sql: construct and maintain in-memory state to tack tenants sql-zcfgs/kv-scfgs

[arulajmani]

Is your feature request related to a problem? Please describe.

As part of #66348, every sql pod will be responsible for reconciling sql-zcfgs to kv-scfgs. To do so, the sql pod would maintain an in-memory structure that captures descriptor hierarchies, the tenant's sql-zcfg information, the desired kv-scfgs state, and the actual kv-scfgs state. I'm tentatively calling this ZoneConfigReconciliationState.

The ZoneConfigReconciliationState should be able to react to zone configuration/descriptor updates. This would allow us to hook this thing up to a rangefeed on these tables later.

Additionally, the ZoneConfigReconciliationState should also be able to hydrate zone configurations by following up inheritance chains, construct span configurations, and find deltas between two sets of span configs.

[arulajmani]

Talked to @ajwerner and I don't think we need to store all of this state in memory. Instead, we should get a transactional view of these things by reading from system.zones and system.descriptors instead. Also realized that the desired kv-scfgs and actual kv-scfgs should be done via RPCs instead of an in-memory diff that @irfansharif and I discussed earlier.