Allow `BitcoinAddress` to represent addresses with Regtest HRP prefix

[fdefelici]

Currenly BitcoinAddress type doesn't support regtest bech32 and bech32m format.

This limitation has been highlighted during PR #6329 with a specific integration tests.
(more details in #6329 (comment)).

The idea is to enhance BitcoinAdress to support these regtest format.

This is also preparatory to completing PR #6329 (although not blocking).