Add branch switcher to pipeline editor empty state

What does this MR do?

For #327673 (closed)

This adds the branch switcher in the empty state of the pipeline editor, allowing the user to switch between branches with and without a CI config.

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

State	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
!59217 (merged)	Fetch the branch list from GraphQL (when query is available)
you are here 👉	Special Case: add branch switcher to empty state

Local Testing

Since the query for fetching the branch list is not yet integrated into the feature, the code relies on a client resolver to show a list of branches. When testing locally, make sure to:

• Enable the :pipeline-editor-branch-switcher feature flag
• Make sure to test with a branch included in the resolver, or add your own test branch in pipeline_editor/graphql/resolvers.js.

Screenshots

Switching Between Empty State and Branch with CI Config

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 feature is hidden 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