Add the branch switcher to the pipeline editor

Problem to solve

Once #321652 (closed) is resolved, we should be in a better position to add the branch switcher component.

We don't need all the bells and whistles at this stage, just whatever we can get done with our current backend and UI library.

Design

🔗 Figma Specs

Documentation

We should be sure to add to the Pipeline Editor documentation with this new functionality.

Availability & Testing

What does success look like, and how can we measure that?

We should be able to:

1. Load the pipeline editor and see that we are on the default branch
2. Load a list of available branches within the toggle.
3. Select one of those branches to allow us to see edit the config for that branch.