Support the rest of available API endpoints

Higher priority:

• https://docs.gitlab.com/ee/api/protected_branches.html
• https://docs.gitlab.com/ee/api/project_level_variables.html (#17 (closed))
• https://docs.gitlab.com/ee/api/pipeline_schedules.html
• https://docs.gitlab.com/ee/api/protected_tags.html (#19 (closed))
• https://docs.gitlab.com/ee/api/remote_mirrors.html (gitlab-org/gitlab#199709)
• https://docs.gitlab.com/ee/api/projects.html#push-rules

Other available:

• https://docs.gitlab.com/ee/api/project_badges.html
• https://docs.gitlab.com/ee/api/deploy_keys.html (gitlab-org/gitlab#419099)
• https://docs.gitlab.com/ee/api/freeze_periods.html
• https://docs.gitlab.com/ee/api/merge_request_approvals.html
• https://docs.gitlab.com/ee/api/project_job_token_scopes.html
• https://docs.gitlab.com/ee/api/boards.html
• https://docs.gitlab.com/ee/api/projects.html#configure-pull-mirroring-for-a-project (gitlab-org/gitlab#6889)

Accept or return a token on creation time which is not possible to get later:

• https://docs.gitlab.com/ee/api/projects.html#hooks (gitlab-org/gitlab#349212 (closed))
• https://docs.gitlab.com/ee/api/pipeline_triggers.html (token can be retrieved only by the creator of a trigger)
• https://docs.gitlab.com/ee/api/deploy_tokens.html (token is available only at creation time)
• https://docs.gitlab.com/ee/api/project_access_tokens.html (token is available only at creation time)
• https://docs.gitlab.com/ee/api/pages_domains.html (for non-automatic keys)

Unsure if we should support it:

• https://docs.gitlab.com/ee/api/members.html
• https://docs.gitlab.com/ee/api/feature_flags.html
• https://docs.gitlab.com/ee/api/deployments.html
• https://docs.gitlab.com/ee/api/environments.html
• https://docs.gitlab.com/ee/api/protected_environments.html
• https://docs.gitlab.com/ee/api/runners.html
• https://docs.gitlab.com/ee/api/cluster_agents.html
• https://docs.gitlab.com/ee/api/error_tracking.html
• https://docs.gitlab.com/ee/api/milestones.html
• https://docs.gitlab.com/ee/api/integrations.html

Feel free to contribute support for them, if you need them.