Fetch and search available branches in pipeline editor

What does this MR do?

For #321655 (closed)

This integrates the getAvailableBranches backend query to the branch switcher in the pipeline editor. It should now fetch the list of available branches for the project and allow the user to search for specific branches.

Branch Switcher Feature

The breakdown of the implementation for the whole branch switcher feature is as follows:

Link	Status	Progress
!57562 (merged)	✔	Add dropdown UI to the pipeline editor
!57941 (merged)	✔	When clicking on a branch, switch to that branch and load the correct data
!60465 (merged)	✔	Add branch switcher to empty state
	you are here 👉	Fetch the branch list from GraphQL

MR Implementation

There is an existing branch selector in the web IDE, but it uses vuex which the pipeline editor is not set up for, so we're using the existing dropdown UI components instead.

It might be helpful to review the MR by commit. The implementation is broken down as follows:

Commit	Progress
2ae15055	Remove client resolver for getAvailableBranches
b21991c	Use getAvailableBranches query to fetch branches
a2d13c89	Get total number of branches for the project
9b31c5bc	Paginate* results when fetching branches
57ffd900	Allow user to search for specific branches

* Results are paginated when the searchTerm is empty (i.e. when we try to fetch all branches), in consideration of projects with a large amount of branches like GitLab.

Local Testing

The feature is under the :pipeline_editor_branch_switcher feature flag, so please enable it first when testing.

Go to CI/CD > Editor and check that the branch switcher works in the pipeline editor!

Screenshots

Search Behavior

Searching Branches

Pagination Behavior

Infinite Scroll

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this feature is behind a feature flag.
• [-] 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