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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proto/caosdb/file_transmission/v1alpha1/file_transmission.proto

@@ -86,15 +86,23 @@ enum TransmissionStatus {
 // TODO: This is very similar to the Hash message in the entity API. We should discuss
 //       if and how they can be merged.
 enum ChecksumAlgorithm {
+  // The checksum algorithm is unspecified.
   CHECKSUM_ALGORITHM_UNSPECIFIED = 0;
+  // The MD5 checksum algorithm.
   CHECKSUM_ALGORITHM_MD5 = 1;
+  // The SHA-1 checksum algorithm.
   CHECKSUM_ALGORITHM_SHA1 = 2;
+  // The SHA-256 checksum algorithm.
   CHECKSUM_ALGORITHM_SHA256 = 3;
+  // The SHA-512 checksum algorithm.
   CHECKSUM_ALGORITHM_SHA512 = 4;
 }
 
+// Checksum contains the checksum value and the algorithm used to compute it.
 message Checksum {
+  // The checksum value.
   string checksum = 1;
+  // The algorithm used to compute the checksum.
   ChecksumAlgorithm checksum_algorithm = 2;
 }
 
@@ -116,20 +124,25 @@ message FileInformation {
 // Register a file upload for a list of files. The server will respond with a
 // registration id and URLs to which the files can be uploaded.
 message RegisterFileUploadRequest {
+  // Information about the files to be uploaded.
   repeated FileInformation file_information = 1;
 }
 
 // The URL to which a file should be uploaded using the given LinkAhead
 // UploadProtocolVersion.
 message UploadLocation {
+  // The URL to which the file should be uploaded.
   string url = 1;
 }
 
 // The version of the LinkAhead upload protocol. This is currently identical to
 // the TUS protocol version.
 message UploadProtocolVersion {
+  // The major version number of the upload protocol.
   int32 major = 1;
+  // The minor version number of the upload protocol.
   int32 minor = 2;
+  // The patch version number of the upload protocol.
   int32 patch = 3;
 }
 
@@ -142,7 +155,9 @@ message RegisterFileUploadResponse {
   string registration_id = 2;
   // The server's transmission settings for the upload.
   FileTransmissionSettings upload_settings = 4;
+  // The version of the LinkAhead upload protocol.
   UploadProtocolVersion version = 5;
+  // The locations where the files should be uploaded.
   repeated UploadLocation locations = 6;
 }