ADR for switching to ABCI++ early

[evan-forbes]

Some important features will require validators being able to perform checks on proposed blocks before voting, along with being able to manipulate the block data for the proposal block directly from the app. While we don't have to use, or wait for, all of ABCI++ to be finished in tendermint/cosmos-sdk, we will have to use something similar to #7091 and what is already in the ABCI++ branch

This change reaches the threshold of requiring an ADR, so we should write one that describes our implementation options, and ultimately leads to a decision for our path to testnet.