[API] Approval rule endpoints

See merge request gitlab-org/gitlab-ee!8769

[EE] Reduce diff with CE in Groups::UpdateService

See merge request gitlab-org/gitlab-ee!9426

[EE] Reduce the diff with CE in Ci::CreatePipelineService

See merge request gitlab-org/gitlab-ee!9378

Docs: add index for project operations

Closes #56115

See merge request gitlab-org/gitlab-ce!24565

Docs: add index for project operations

Closes gitlab-ce#56115

See merge request gitlab-org/gitlab-ee!9253

Reduce diff with EE in Search::GlobalService

See merge request gitlab-org/gitlab-ce!24914

[EE] Reduce diff with CE in Search::GlobalService

See merge request gitlab-org/gitlab-ee!9427

Resolve "Allow SSO enforcement in group settings for GitLab.com"

Closes #5291

See merge request gitlab-org/gitlab-ee!9240

Neded for client side validation
and needed to know if a MR rule should be editable or not.

Use existing project PUT to update

fallback_approvals_required should be checked in conjunction with
code owner rules (if any)

Specs are based from existing approvable spec.

Update spec

No longer need to consider overall_approvals_required

Fix spec so fallback path is tested

It is helpful to not duplicate these paths in the FE.

For the `approvals_settings` response.
It was needed because we automatically populate the MR `rules` from
the project if necessary, but there was no way for the edit UI to know
if these were project rules or MR rules.

Now endpoints are under approval_settings scope.

This excludes code owner rule

Now endpoints are under approval_settings scope.
Add PUT /projects/:id/approval_settings for updating
overall approvals_required.

Combine checking query

Allow assigning project rule for MR rule

When no MR approval rule, we don't validate as project rule will be
used.

This is to recompute the new overall_approvers
In this method, the following calls would cause a change in the
database which needs resetting the cache:

- super (which updates MR)
- sync_approval_rules (which updates rule record's approvals_required)
- cleanup_approvers (which removes any approver not mentioned in params)

resetting cache must be done after those three calls,
so moving some calls earlier is required.
However this would not affect outcome.

Accepts nested attribute for rules