Implement DB `use_replicas_for_read_queries`

What does this MR do?

The second part of gitlab-com/gl-infra/scalability#1005 (closed). Should be deployed after !59086 (merged).

This MR implements use_replicas_for_all_reads to redirect all read queries to the replicas, regardless of primary stickiness. This would help optimizing the expensive queries, forcefully pushing them to the replicas. This does not affect write or ambiguous queries.

::Gitlab::Database::LoadBalancing::Session.current.use_primary!
::Gitlab::Database::LoadBalancing::Session.current.use_replicas_for_read_queries do
  User.find_by(email: 'a@a.com') # This goes to a replica, regardless of recent stickiness
end

Screenshots (strongly suggested)

Not really. This is a new interface at the load balancing layer.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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.
• Tested in all supported browsers
• 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