#42: feat(async): support topology view

Track cluster topology (instances, replicasets) on the client, modeled after Picodata's topology notifications RFC: https://git.picodata.io/core/picodata/-/blob/master/doc/adr/2025-09-12-pgproto-topology-notifications.md

Picodata doesn't push topology changes as NOTIFY events yet (core/picodata#1306), so for now the client polls Picodata's system tables directly, through a TopologySource abstraction. Once Picodata starts pushing real events, consider to replace topology source with listeting picodata notifications

Currently the pool only reads this topology -- it does not change its connection. Acting on topology changes (refilling pool connections, bucket-aware routing, etc.) is intentionally left for follow-up work.