Extend Upload API for resumable downloads

Summary

Related to #24. This MR changes the way how files are uploaded. The files to be uploaded are merely registered with the server, which responds with URLs, to which the files are supposed to be transmitted using the specified LinkAhead File Upload Protocol Version. This enables using the TUS protocol for uploading files via HTTPS.

Focus

• The file transmission API is split from the entity API into its own proto file (v1alpha). The entity API version is incremented to v2alpha1 (v2alpha1 is currently identical to v1 but lacking the file transmission API).
• The main changes to the file transmission API are in commit 014805af

There are two open questions where a decision should be made (marked in the proto files with TODO)

1. The FileInformation message in the file transmission API is very similar to the FileDescriptor message in the entity API with the difference that it has no relation to an entity and is purely filesystem-oriented. Maybe we should re-use FileInformation s part of the entity API?
2. Checksum and ChecksumAlgorithm of the file transmission API are very similar to the Hash message in the entity API. We should discuss if and how they can be merged.

Test Environment

How to set up a test environment for manual testing?

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