`regtest-launcher`: implement faucet

[dorianvp]

In the current setup, the miner's keys are exposed for consumers to import and directly manage those funds. However, this means there's a requirement for clients to correctly handle immature coinbase spends, which some clients do not. This is in some situations a no-go when developing lightclients.

An approach could involve hiding the lightclient behind a feature flag. It's also important to consider what changes will need to be made to the current user flow: the miner address could be hidden, and the recipient could be the address for which we expose key material.

[Timi16]

Please is this Issues still open want to implement this