feat(datastore): introduce database load balancer entity

What does this MR do?

This is the first step towards DLB: Implement load balancing at the connection... (#1276 - closed).

Introduces a new datastore.LoadBalancer entity that supports holding separate DB handlers for primary and replica hosts. This is meant to replace the direct use of datastore.DB (which is encapsulated in the former, one instance per host).

A basic round-robin replica selection was implemented in this first iteration. The load balancer defaults to the primary handler if database load balancing is not enabled or no replicas are configured.

For now, I'm explicitly targeting the primary handler for every use case across the whole codebase (regardless of load balancing configurations), so the as-is behavior is safely preserved until we start rolling out the feature gradually (endpoint by endpoint).

The next step is to introduce support for configuring load balancing replicas through the database.loadbalancing.hosts configuration parameter.

Author checklist

• Feature flags
  • Added feature flag:
  • This feature does not require a feature flag
• I added unit tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines
• For database changes including schema migrations:
  • Manually run up and down migrations in a postgres.ai production database clone and post a screenshot of the result here.
  • If adding new queries, extract a query plan from postgres.ai and post the link here. If changing existing queries, also extract a query plan for the current version for comparison.
    • I do not have access to postgres.ai and have made a comment on this MR asking for these to be run on my behalf.
  • Do not include code that depends on the schema migrations in the same commit. Split the MR into two or more.
• Ensured this change is safe to deploy to individual stages in the same environment (cny -> prod). State-related changes can be troublesome due to having parts of the fleet processing (possibly related) requests in different ways.

Reviewer checklist

• Ensure the commit and MR tittle are still accurate.
• If the change contains a breaking change, apply the breaking change label.
• If the change is considered high risk, apply the label high-risk-change
• Identify if the change can be rolled back safely. (note: all other reasons for not being able to rollback will be sufficiently captured by major version changes).

If the MR introduces database schema migrations:

• Ensure the commit and MR tittle start with fix:, feat:, or perf: so that the change appears on the Changelog

If the changes cannot be rolled back follow these steps:

• If not, apply the label cannot-rollback.
• Add a section to the MR description that includes the following details:
  • The reasoning behind why a release containing the presented MR can not be rolled back (e.g. schema migrations or changes to the FS structure)
  • Detailed steps to revert/disable a feature introduced by the same change where a migration cannot be rolled back. (note: ideally MRs containing schema migrations should not contain feature changes.)
  • Ensure this MR does not add code that depends on these changes that cannot be rolled back.