Support coinbase outputs

[shesek]

Typically needed for regtest chains.

See sparrowwallet/sparrow#265

[GambolingPangolin]

Here is a similar change in the Electrum Personal Server repo: chris-belcher/electrum-personal-server#64

FWIW electrum tracks coinbase transactions. It shows an "unmatured" balance as needed and does not include immature coinbase transactions in the spendable set.

[VzxPLnHqr]

For what it is worth, I also ran into this when trying to get a local sparrow + regtest (bitcoind) up and running. I used an address from the regtest sparrow wallet as the bitcoin-cli generatetoaddress target and was unable to see any of the transactions in sparrow.