tendermint-rpc: Improve Query API

[romac]

Description

I would like to improve the tendermint_rpc::Query API in two ways which would be useful for Hermes:

1. Make the event_type and conditions fields of Query public. This would allow us to introspect, modify and evaluate queries on the Hermes side, something which we find ourselves needing right now to work around a behavior of the Tendermint indexer.
2. Add a fn key(&self) -> &str method to Condition which would return the key that this condition applies to. The use case is the same as (1), where we need to partition conditions by keys and only evaluate the relevant ones.

Definition of "done"

The two suggestions above are implemented.