Add API support for retrieving deployed merge requests

What does this MR do?

This MR adds API support for retrieving merge requests deployed in a deployment. It reuses the parameters defined in the Merge Requests API. Also, we are introducing a new endpoint in the Deployment API: :id/deployments/:deployment_id/merge_requests

As a result, some refactoring was done in order to be consistent between the two APIs.

The new endpoint uses the MergeRequestsFinder to retrieve merge requests. This finder was then updated (and tests were added consequently)

Screenshots

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

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

Related to #35739 (closed)