feat: dropbox source

[nicoloboschi]

• New dropbox-source

- type: "dropbox-source"
                                    id: "step1"
                                    configuration:
                                        access-token: sl.xxxxx
                                        path-prefix: /langstream-test
                                        state-storage: s3
                                        state-storage-s3-bucket: "test-state-bucket"
                                        state-storage-s3-endpoint: "%s"
                                        deleted-objects-topic: "deleted-objects"

• access-token for the app (Follow this)
• path-prefix for reading from a different root path
• extensions , list for extensions to filter in (by defaut all are included)