Api endpoints for UTXOs differ between devnet and test/mainnet

[radicleart]

Linked to this issue stacks-network/sbtc#230

Mempool api on bitcoins regtest network does not support the /:address/utxo endpoint

The bridge api requires this endpoint to read utxo's to build PSBTs.

This means that to work on regtest the bridge api must either;

1. connect to a full node indexer directly (electrs, electrum etc) running locally
2. or build the UTXO set by first importing the address into Bitcoin core

Running the indexer locally has proven challenging. Electrs is included in the devenv local network for the sBTC project but this has proven unstable.

The Leather wallet also requires the /utxo endpoints and so the solution for the Bridge api needs to be consistent with that for the wallet.

Note: Leather looks for endpoints like /:address/utxo on port 3002 for sBTC local network or port 18443 for the devnet local network.