Implement "mining" blocks in regtest mode

[nchashch]

There is already an implementation of this in the bip300301_wallet repository, just move it here.

[torkelrogstad]

What's your opinion on the idea brought up in this discussion in the protos repo?

LayerTwo-Labs/cusf_sidechain_proto#5 (comment)

I've implemented functionality for adding coinbase messages to the signet miner here: LayerTwo-Labs/bitcoin-patched#6

Doing this approach in regtest as well could be a good way of testing that it works properly. And it would let us omit the generate RPC

[nchashch]

It would be easier to copy over the block construction from bip300301_wallet implementation.

[torkelrogstad]

I'm pretty sure that won't work on signet, though?

[nchashch]

Yes, but we need to get it to work with regtest first. We can worry about signet later.

Because all the other things we need to integrate Thunder and Enforcer depend on this working.