Generate Avro schemas from a Dataset

The [MVP](https://github.com/confluentinc/vscode/issues/2323) supports generating a Dataset file from an Avro schema. However, with the introduction of Interactive Dataset Design, the Dataset file becomes the central source of truth for mock data design. Since Datasets evolve over time, it should also be possible to generate Avro schemas from them.

Proposed behavior:
- Add a CodeLens option to Dataset files for generating an Avro schema.
- On selection, call the Medusa /v1/schemas/avro/convert endpoint with the Dataset file.
- Prompt the user to select a save location on their machine.
- Save the returned contents as an *.avsc file.