Move GitLab output rendering to `ruff_db` (#20117)
## Summary
This PR is a first step toward adding a GitLab output format to ty. It
converts the `GitlabEmitter` from `ruff_linter` to a `GitlabRenderer` in
`ruff_db` and updates its implementation to handle non-Ruff files and
diagnostics without primary spans. I tried to break up the changes here
so that they're easy to review commit-by-commit, or at least in groups
of commits:
- [preparatory changes in-place in `ruff_linter` and a `ruff_db`
skeleton](https://github.com/astral-sh/ruff/pull/20117/files/0761b73a615537fe75fc54ba8f7d5f52c28b2a2a)
- [moving the code over with no implementation changes mixed
in](https://github.com/astral-sh/ruff/pull/20117/files/0761b73a615537fe75fc54ba8f7d5f52c28b2a2a..8f909ea0bb0892107824b311f3982eb3cb1833ed)
- [tidying up the code now in
`ruff_db`](https://github.com/astral-sh/ruff/pull/20117/files/9f047c4f9f1c826b963f0fa82ca2411d01d6ec83..e5e217fcd611ab24f516b2af4fa21e25eadc5645)
This wasn't strictly necessary, but I also added some `Serialize`
structs instead of calling `json!` to make it a little clearer that we
weren't modifying the schema (e4c4bee35d39eb0aff4cae4a62823279b99c6d9c).
I plan to follow this up with a separate PR exposing this output format
in the ty CLI, which should be quite straightforward.
## Test Plan
Existing tests, especially the two that show up in the diff as renamed
nearly without changes
