server/v2 Indexer Integration

[aaronc]

To support indexing in server/v2 we need to setup locations for appdata.Listener callbacks to be invoked. Two possible locations for this are:

1. https://github.com/cosmos/cosmos-sdk/blob/main/server/v2/cometbft/streaming.go
2. https://github.com/cosmos/cosmos-sdk/blob/main/server/v2/stf/stf.go

Integration into cometbft/streaming.go might be easier but IMHO is less ideal and less performant. The ideal integration spot IMHO would be directly in stf and a draft PR was created here: #21273.

Ideally, #21312 should be completed first and then a first class integration of appdata.Listener can be added directly to stf.

[aaronc]

I don't think this should have been closed. That PR as far as I know was just one part of the implementation.