feat: add support for gitea platform

[ppmathis]

This pull requests adds support for the Gitea platform, which is available as a self-hosted solution as well as on gitea.com operated by DiDi Cloud. This specific implementation supports all Renovate features aside from these caveats:

• Adding reviewers to a PR is not supported, as Gitea simply does not have an API for this yet. You can find the upstream issue here.
• Ignoring a PR does not work, as Gitea does not expose the branch name of a PR after said branch has been deleted. You can find the upstream issue here, which has been opened by myself. To warn the user about this behavior, all Renovate pull requests are marked as Immortal and the Renovate Ignore Notification comment gets suppressed when closing a PR.

The implementation has 100% test coverage and was successfully tested against a few real repositories on a self-hosted Gitea server running 1.11.0. It is split across three files:

• index.ts does all the actual hard work as with any other currently featured platform
• gitea-helper.ts does the actual API calls and exposes them using typed functions
• gitea-got-wrapper.ts as a wrapper for the got library

During the implementation I've noticed that setBaseBranch has been incorrectly type-hinted in lib/platform/common.ts, as the baseBranch was not flagged as optional despite being supported and required by the current platforms / test suites. This has been fixed in the separate commit e39d221, which can be found inside this PR as well.

Thank you in advance for reviewing this contribution.

Closes #4042

[github-actions]

🎉 This PR is included in version 19.141.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[lunny]

I sent a PR go-gitea/gitea#10413 to fix the second problem.