Databases: Allow kaspad to run with an optional `txindex` database

[D-Stacks]

Ideally this should run in a similar fashion to the utxoindex. It shoud store the txID as the key, and the accepting blockhash as the value. The accepting blockhash can be used to get blockdata assoiated with the txID from the consensus database. Storing the whole block will probably cause too much data bloat under heavy network usage, since each block will be stored multiple times for each tx in it.

This can be used to query tx data via rpc, calculate confirmations, and so on.