Support uploading files for server-side scripting via gRPC

Summary

The REST API supports uploading files along with a request for SSS. This should ideally also be possible via gRPC.

Expected Behavior

Upload files for SSS via gRPC is not supported.

Actual Behavior

What did the software actually do?

Specifications

• Version: 0.14-dev
• Platform: any

Possible fixes

See also new upload service via TUS (e.g. #354)