Extract indexing exclusive locking to be used by FKs too

What does this MR do and why?

Related to #390651 (closed) and #386242 (closed)

We need to extract this lock to a new class/key because we will need to reuse it for async foreign key validations because we don't want to have the indexing process and the validation one running at the same time.

Follow-up for FK usage: !111330 (merged)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.