feat(repo/changelogs): allow user configuration of source url (#16873)

docs/usage/configuration-options.md

lib/config/options/index.ts

lib/config/types.ts

lib/workers/repository/update/pr/changelog/common.spec.ts

lib/workers/repository/update/pr/changelog/common.ts

lib/workers/repository/update/pr/changelog/github.spec.ts

lib/workers/repository/update/pr/changelog/gitlab.spec.ts

lib/workers/repository/update/pr/changelog/index.ts

lib/workers/repository/update/pr/changelog/source-github.ts

lib/workers/repository/update/pr/changelog/source-gitlab.ts