feat(apiv2): stream and progress endpoints for API v2

[carlobortolan]

Motivation and Context

See #1415.

Description

• Add stream and progress related endpoints (see stream.go and progress.go) to get details, playlists, sections, subtitles and thumbnails for a stream and progresses
• Rename GetLiveStreams APIv2 endpoint to GetLiveCourses
• Add authorizeUserForStreamCourse method to check if user is allowed to access a specific stream (<- open for feedback/alternative approaches)
• Clean up protofile and remove duplicate /api/v2 path in routes for HTTP proxy
• Add google/api/httpbody.proto to support custom response body filetypes (e.g., text/vtt for subtitles or image/jpeg for the thumbnails)
• Fix duplicate Notification message bug (see related feat(runner): heartbeat #1493 and feat(apiv2): bookmark and notification endpoints for API v2 #1492)

Steps for Testing

Test gRPC API:

1. Fork postman.com/tumdev/team-workspace/collection/66dba0f1406ecba7a2d44836 or use apiv2.proto
2. Try out different requests

Test HTTP/REST proxy:

1. Open localhost:8081/api/v2/docs
2. Try out different requests

Screenshots

List of new endpoints:

[joschahenningsen]

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