feat: add support for gitea platform (#5509)

docs/development/self-hosting.md

docs/usage/modules/platform.md

lib/constants/platforms.ts

lib/platform/common.ts

lib/platform/gitea/README.md

lib/platform/gitea/gitea-got-wrapper.ts

lib/platform/gitea/gitea-helper.ts

lib/platform/gitea/index.ts

lib/util/got/auth.ts

test/platform/__snapshots__/index.spec.ts.snap

test/platform/gitea/__snapshots__/index.spec.ts.snap

test/platform/gitea/gitea-got-wrapper.spec.ts

test/platform/gitea/gitea-helper.spec.ts

test/platform/gitea/index.spec.ts

test/platform/index.spec.ts