Create pending status check responses per MR

What does this MR do and why?

What

Create pending status check responses when an MR is created

Why

Required in preparation for #388725 (closed)

Screenshots or screen recordings

No changes to UI

How to set up and validate locally

1. Simulate a SaaS instance (export GITLAB_SIMULATE_SAAS=1)

2. Enable :pending_status_check_responses FF

3. Under Project > Settings > Merge Requests > Status Checks

   Add some example status checks (see example screenshot below):

   

4. Create an MR (Edit the README.md of any project using the WebIDE and create an MR)

5. Check that pending status check responses have been created in the db

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

Related to #413723 (closed)