Add example using a configurable LangChain document loader (#543)

examples/applications/langchain-document-loader/.langstreamignore

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+# .langstreamignore file inspired by https://github.com/github/gitignore/blob/main/Python.gitignore
+
+# Byte-compiled / optimized / DLL files
+__pycache__/
+*.py[cod]
+*$py.class
+
+# C extensions
+*.so
+
+# Distribution / packaging
+.Python
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+wheels/
+share/python-wheels/
+*.egg-info/
+.installed.cfg
+*.egg
+MANIFEST
+
+# These folders hold the libs built for the target
+# and we need them in the package
+!python/lib/
+!java/lib/
+
+# PyInstaller
+#  Usually these files are written by a python script from a template
+#  before PyInstaller builds the exe, so as to inject date/other infos into it.
+*.manifest
+*.spec
+
+# Installer logs
+pip-log.txt
+pip-delete-this-directory.txt
+
+# Unit test / coverage reports
+htmlcov/
+.tox/
+.nox/
+.coverage
+.coverage.*
+.cache
+nosetests.xml
+coverage.xml
+*.cover
+*.py,cover
+.hypothesis/
+.pytest_cache/
+cover/
+
+# Translations
+*.mo
+*.pot
+
+# Django stuff:
+*.log
+local_settings.py
+db.sqlite3
+db.sqlite3-journal
+
+# Flask stuff:
+instance/
+.webassets-cache
+
+# Scrapy stuff:
+.scrapy
+
+# Sphinx documentation
+docs/_build/
+
+# PyBuilder
+.pybuilder/
+target/
+
+# Jupyter Notebook
+.ipynb_checkpoints
+
+# IPython
+profile_default/
+ipython_config.py
+
+# pyenv
+.python-version
+
+# pipenv
+Pipfile.lock
+
+# poetry
+poetry.lock
+
+# pdm
+pdm.lock
+.pdm.toml
+
+# PEP 582; used by e.g. github.com/David-OConnor/pyflow and github.com/pdm-project/pdm
+__pypackages__/
+
+# Celery stuff
+celerybeat-schedule
+celerybeat.pid
+
+# SageMath parsed files
+*.sage.py
+
+# Environments
+.env
+.venv
+env/
+venv/
+ENV/
+env.bak/
+venv.bak/
+
+# Spyder project settings
+.spyderproject
+.spyproject
+
+# Rope project settings
+.ropeproject
+
+# mkdocs documentation
+/site
+
+# mypy
+.mypy_cache/
+.dmypy.json
+dmypy.json
+
+# Pyre type checker
+.pyre/
+
+# pytype static type analyzer
+.pytype/
+
+# Cython debug symbols
+cython_debug/
+
+# PyCharm
+.idea/

examples/applications/langchain-document-loader/README.md

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+# LangChain Source
+
+This sample application shows how to use LangChain in a Python agent.
+The application is a Source that uses a configurable LangChain to load documents and then uses a configurable LangChain text splitter to chunk the document and send the chunks to a Kafka topic.
+
+## Configure the pipeline
+
+Update the pipeline file and set the loader and splitter class names and their parameters.
+Set the load-interval-seconds to the number of seconds between each load of documents. 
+The load-interval-seconds value -1 (default) means load once.
+
+## Deploy the LangStream application
+
+```
+./bin/langstream apps deploy test -app examples/applications/langchain-document-loader -i examples/instances/kafka-kubernetes.yaml
+```
+
+## Consume from the Gateway Consumer
+```
+./bin/langstream gateway consume test consume-output
+```

examples/applications/langchain-document-loader/gateways.yaml

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+#
+#
+# Copyright DataStax, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+gateways:
+  - id: consume-output
+    type: consume
+    topic: output-topic

examples/applications/langchain-document-loader/pipeline.yaml

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+#
+# Copyright DataStax, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+name: "LangChain document loader"
+topics:
+  - name: "output-topic"
+    creation-mode: create-if-not-exists
+pipeline:
+  - name: "Load documents and chunk them with LangChain"
+    type: "python-source"
+    output: "output-topic"
+    configuration:
+      className: langchain_document_loader.LangChainDocumentLoaderSource
+      load-interval-seconds: 3600
+      loader-class: WebBaseLoader
+      loader-args:
+        web-path: ["https://langstream.ai/"]
+      splitter-class: RecursiveCharacterTextSplitter

examples/applications/langchain-document-loader/python/Pipfile

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+[dev-packages]
+pyYaml = "*"
+langstream-ai = "*"
+langchain = "*"
+pytest = "*"
+tox = "*"