server: need to maintain sql_instances in all cases

[knz]

Describe the problem

The system.sql_instances table are only maintained by SQL servers that were operating in "pod" mode, i.e. not in mixed KV and SQL process nodes, where KV-level liveness and gossip provides an alternative means of node discovery that can be used by the SQL layer when searching for other SQL instances.

However this inconsistency makes writing correct remote-node discovery and interaction SQL-level code difficult: in some cases such code needs to consult the instances list, and in some the KV liveness store, which when combined with complexities of doing so around initialization, dependency-injection, etc can become hard to maintain.

Additionally such a design precludes a cluster where some SQL instances are in mixed KV nodes and some are not, as the non-KV nodes would have no way discover the KV ones. Such deployments are not currently possible but could be in the future.

Expected behavior

Instead, this change enabled maintenance of the sql_instances table by all SQL servers, whether running in their own processes or embedded in a KV storage node process.

Epic: CRDB-14537

Jira issue: CRDB-23594