Rake task to reindex an index

What does this MR do?

Relates to: #230829 (closed)

Currently, pg_repack is manually used to rebuild indexes that become bloated on GitLab.com. This process needs to be automated, in a way that can be run on self-managed installations in addition to GitLab.com.

This MR introduces a new rake task that can be used to concurrently reindex an existing index, which will be executed automatically by the application.

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

Availability and Testing

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team