Added proto3 optional flag for `protoc` custom build in `brane-tsk`

[marinoandrea]

• Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines

• [ ] Tests for the changes have been added (for bug fixes / features)

• [ ] Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) This simply adds the --experimental_allow_proto3_optional flag to the protoc invocation in brane-tsk custom build.

• What is the current behavior? (You can also link to an open issue here) https://github.com/epi-project/brane/issues/33

• What is the new behavior (if this is a feature change)? The build does not error out.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?) Not that I am aware of.

[Lut99]

Wow, thanks :)

However, I should have told you yesterday - the develop branch holds the latest changes, and in theory I should have removed the protobuf compiler (protoc) dependency altogether. Although this would probably fix the dependency issue on Debian-based distros (which ship with a protoc compiler that requires this change), I'd rather avoid this extra dependency if I can. Moreover, because I only use these protobuf definitions for inter-Rust communication, it's more practical to define them in Rust directly so we can simply tag existing structs instead of re-defining them in protobuf.

Let me know if you get the same error in the develop branch, or, alternatively, let me bring the main branch up-to-date and test it myself. If the issue persists we can merge this issue, or otherwise I think I'll close it :# But thanks a lot for you work, though!

[marinoandrea]

I see, will close the PR then.