Extracting lint-related stuff into separate proto file. It
is required in order to have proper working proto-linter.
Previously it was using compiled files for verification and
it fails in some cases
(https://gitlab.com/gitlab-org/gitaly/-/jobs/459024976).
lint.proto extracted from shared.proto and contains
lint-related declarations. New task `proto-lint` added to
compile source code that is required by `protoc-gen-gitaly`.
`protoc-gen-gitaly` fixed to use proper proto source data.
Regeneration of all proto-related files.