Update Zoekt index when default branch change

What does this MR do and why?

Related to #389763 (closed)

• Add a new event handler/worker to queue indexing for zoekt enabled projects when the default branch changes
• Adds two new factories

Screenshots or screen recordings

N/A

How to set up and validate locally

1. setup elasticsearch and zoekt in gdk, configure a root namesapce for zoekt (I chose Flightjs)
2. create a new branch or pick an existing non-default branch
3. commit a new file to that branch
4. change the default branch to be that branch
5. verify that search finds the new file and is searching the new default branch
6. verify that zoekt search is being used (it should say Exact Code search powered by Zoekt). also in the performance bar, you should see no es calls and should see some zkt calls

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.