[OPIK-48] e2e tests

[alexkuzmik]

This PR adds the first batch of e2e tests between the SDK and the backend.

Currently covered happy flows:

1. Creating and populating a dataset
2. Manual trace and span creation via Opik class.
3. Automatic functions tracing via @track decorator
4. Feedback logging for spans and traces via Span, Trace and Opik classes.
5. Experiment creation via evaluate function call.
6. Added get_trace_content() and get_span_content() methods to Opik class, they return dataclass instances with the corresponding trace/span data retrieved from the backend.

To run tests:

pytest tests/e2e
If you need to specify some env vars, you can do it like that:
OPIK_WORKSPACE=xxx OPIK_API_KEY=yyy pytest tests/e2e

Notes:

1. All e2e tests are stored in tests/e2e
2. Currently, the tests are running against the backend configured. Tests are NOT a part of the CI yet.
3. Tests log traces to e2e-tests project
4. Nothing is deleted after the test run except for the dataset (we'll need to discuss whether we need this deletion or no and then proceed with the rest of the data depending on what we decide)