Enable optional fields for Protocol Buffers older than v3.15.0

[upbqdn]

Motivation

Close #8278.

PR Author Checklist

Check before marking the PR as ready for review:

• Will the PR name make sense to users?
• Does the PR have a priority label?
• Is the documentation up to date?

Solution

• Enable the experimental_allow_proto3_optional flag for protoc.

Reviewer Checklist

Check before approving the PR:

• Does the PR scope match the ticket?
• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
• Are all the PR blockers dealt with?
  PR blockers can be dealt with in new tickets or PRs.

And check the PR Author checklist is complete.

[oxarbitrage]

That is good enough for me, thanks for the fix!