feat: write git private key (#6321)

docs/usage/self-hosted-configuration.md

lib/config/common.ts

lib/constants/error-messages.ts

lib/platform/bitbucket-server/index.ts

lib/platform/common.ts

lib/platform/git/private-key.spec.ts

lib/platform/git/private-key.ts

lib/platform/git/storage.ts

lib/platform/gitea/index.ts

lib/platform/github/index.ts

lib/platform/github/types.ts

lib/platform/gitlab/index.ts

lib/platform/index.ts

lib/workers/repository/init/apis.ts