Ingest and expose stacker/signer set event data

[zone117x]

PR stacks-network/stacks-core#4269 adds a new event /new_pox_anchor which exposes stacker/signer set data. We need to ingest these and will then be able to expose much of the (non-sBTC related) data requested at hirosystems/explorer#1248

[zone117x]

Somewhat blocked on #1763 -- progress can be made, but integration tests for it require running against epoch3 which isn't yet stable

[zone117x]

The initial ingestion and endpoints for this were implemented in #1873. There are more detailed issues open tracking further work around this area: #1953