Support querying for all events at specific height

[kostko]

Currently it is only possible to subscribe to events via the various Watch* methods exposed by the backends. We should add a new method to each backend's API to retrieve all events at a specific height.

For the Tendermint backend this can be implemented by querying ABCI responses at specific height (using GetBlockResults) and converting that into events (note that some events require additional ABCI state queries).

An alternative (or a later addition as a cache) would be to separately index events in a database (e.g., a Badger database which stores per-height events as <height> <backend-id>) and then just return them when requested. This would also need to take pruning into account to prune old data based on the existing configured consensus state pruning policy.