gRPC API/v2

[carlobortolan]

Is your feature request related to a problem? Please describe.
Currently, GoCast is built on top of a REST API written in Go using the Gin web framework. However, this API is unversioned and, therefore, can't be really used by external services (e.g., the Campus App, Artemis, a future TUM-Live App, etc.).

Describe the solution you'd like
A new, versioned gRPC API called api_v2 which allows for external services (e.g., TUM Campus App, future GoCast App, etc.) to integrate lecture videos and features with a stable API.
While not immediately relevant, gRPC would also have the benefit of other features such as bi-directional streaming or better performance compared to HTTP/REST.

Describe alternatives you've considered
Keep the current system as is.

Additional context
This issue is used to keep an overview of the progress with the new API:

User-related endpoint groups:

• Implement user endpoints
• Implement course endpoints
• Implement stream endpoints
• Implement notification endpoints

Lecturer-related endpoint groups:

• Implement admin endpoints
• Implement token endpoints
• Implement
• TODO

Note

A prototype was developed last year for the iPraktikum GoCast app (see this PR draft) but has not been updated since then. To avoid changing the API used for the GoCast app and avoid having one very large commit, I'd implement each endpoint group as one and to incrementally add merge them to the production system.