PR checks: add with clientcompact tests

[marioizquierdo]

Fixes #315

Update the Github workflow used to verify pull requests.

Specific changes:

• Add clientcompact tests to commit check
• Remove coverage checks. The coverage was for 12% only, but that is because most of the code in the package is auto-generated code. Coverage is not significant for Twirp, because the generator is indirectly tested through the generated files. Twirp reviewers need to manually check that tests are properly covering use-cases anyways.
• Remove pr-checks, keep only commit push checks: because the only difference was the coverage.
• Cleanup Makefile
• Re-generate code with newer protoc compiler

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.