Use AssessEvidence instead of StoreEvidence RPC on Clouditor.

Some refactoring.

Squashed commit of the following:

commit 11ae9a48f6b41c2dc5b3e00de1b808b75cc39013
Author: Anže Žitnik <anze.zitnik@xlab.si>
Date:   Tue Dec 14 11:40:01 2021 +0100

    Change CI script: build and test all branches

commit fe84541d50ffc6b25d5fff94b1781345ec2b548d
Author: Anže Žitnik <anze.zitnik@xlab.si>
Date:   Tue Dec 14 11:33:07 2021 +0100

    Version up

commit b99df078408ea2649ce59cd2d17c247c04c6a992
Author: Anže Žitnik <anze.zitnik@xlab.si>
Date:   Tue Dec 14 11:25:12 2021 +0100

    Update dockerignore: add (v)env folder.

commit 20944e6743ce112d558fb0205a9347a46c17da8b
Author: Anže Žitnik <anze.zitnik@xlab.si>
Date:   Tue Dec 14 10:56:41 2021 +0100

    Refactoring: move all gRPC-generated files to their own package (grpc_gen).

commit f4dce9c9076a1336dc7d0b5e15759b24c4f9bea7
Author: Anže Žitnik <anze.zitnik@xlab.si>
Date:   Tue Dec 14 10:34:05 2021 +0100

    Use AssessEvidence instead of StoreEvidence RPC on Clouditor.

    Added necessary proto files (removed unneeded) and regenerated python code from them. Also added Google APIs dependencies for grpc code generation. Accordingly updated README. Also updated requirements.txt to include some packages for grpc python code generation and for usage of Google APIs.

    Note that with the call to AssessEvidence, we need to provide an AssessEvidenceRequest object (instead of simply Evidence as before). Most changes of existing code are because of this.