gitlab-ci: Run configured linters as done on Phabricator

Provide a backward-compatible way to run linters for projects that have been configured for use with Phabricator/Differential.

Even without Phabricator, arc lint is a good configurable frontend to run per-language linters, so it's not a bad choice even without backward compatibility concerns.

See !14 (closed) for an example of a MR with some non-fatal suggestions.

This is the task that prompted this MR: https://phabricator.apertis.org/T6299

(!10 (merged) is an older version of this MR that was landed by mistake to v2020pre and has been reverted)