WIP: serve: add os-version info exchange

This is to address the point discuted in:

gitlab-shell#609 (comment 1444982614)

TODO:

• add protocol doc
• add tests
• refactor builtin/bugreport.c
• check if calls to git_os_version_sanitized() should be added to more places
• split into multiple commits
• improve commit messages
• add Signed-off-bys