Allow users to get Merge Trains entries via Public API

Important note: This MR should be delivered to gitlab.com asap in order to resume dogfooding for merge train regardless of its milestone

What does this MR do?

In order to address #36146 (closed), we need to create a public API to read Merge Train entity. This entity includes duration param, which can be used for alerting when a duration becomes unexpectedly slow.

Here is a couple of reviewing points:

• We preload associations properly in order to avoid N+1. This is already being tested in spec.
• Since Merge Train entity includes Pipeline and Merge Request entity as well, we check the user permission for it.
• This feature is behind merge_trains_api feature flag, thus we can disable the API when we observe unexpected behavior.

Related #36146 (closed), #32665

API format

Endpoint	params	description
GET api/v4/projects/:id/merge_trains	scope, sort	Get all merge trains of a project

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team