Absorb grpc-proxy into Gitaly

This change absorbs the previously vendored library grpc-proxy into the Gitaly project for breaking changes needed for Praefect requirements. There were slight modifications needed to make the project work within Gitaly:

• Go import paths were rewritten to refer to the new Gitaly location
• Test service code was moved to testdata folder to prevent CI linter warnings
• CI-lint overrides were added to exclude inherited technical debt (see follow up issue #1663 (closed))
• Removal of unused/ineffective code detected by linter