Draft: Fetch merge trains for table and build filtering

What does this MR do and why?

We are working on building a new feature, merge train visualization.

In this MR I fetch merge train and car data for the table and add filtering. In later MRs I will add more to this feature, such as the ability to remove MRs from the train and pagination.

Progress

Part One: !149025 (merged)

Part Two: !151940 (merged)

Part Three: !152400 (merged)

Part Four: You are here

Part Five: TBD

Part Six: TBD

See the entire epic here where I have mapped out this feature build.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Enable merge trains and the FF merge_trains_viz
2. Visit http://gdk.test:3000/namespace/project/-/merge_trains or add an MR to the train and click "View merge details" in the widget on an MR.
3. Ensure you can see the view, with tabs and table
4. Ensure you can filter by target branch where trains are active
5. Ensure table data is displayed correctly
6. Disable the flag and ensure you see a 404 status

Related to #459885 (closed)