Add rake tasks to pause or resume indexing

What does this MR do and why?

Closes #329667 (closed)

In some cases Elasticsearch indexing may make an instance unstable, bringing down Puma. This in turn prevents admins from accessing the UI to pause indexing, leading to extended downtime.

Adds:

• bundle exec rake gitlab:elastic:pause_indexing
• bundle exec rake gitlab:elastic:resume_indexing

Note: I'm waiting on an ultimate license for my local gdk environment, so I'm going to leave this in a draft status until I'm able to test locally. 👍

TODO:

• Update documentation
• Test locally
• Update changelog for EE change

Screenshots or screen recordings

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.