Make elasticsearch bulk parameters configurable

What does this MR do?

These variables strongly affect the amount of load that indexing operations place on the Elasticsearch server. They are useful values to be able to twiddle in both performance and availability contexts.

At present, these only affect gitlab-elasticsearch-indexer behaviour, but we can roll it out to the sidekiq context over time.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• Database guides
• [-] Separation of EE specific content
• New gitlab-elasticsearch-indexer tag cut

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #12375 (closed)