Add Regtest Support to the SDKs

[pacu]

sibling issue of Electric-Coin-Company/zcash-swift-wallet-sdk#1233

Is your feature request related to a problem? Please describe.

Let's add regtest support to the SDKs, so that we can create more flexible scenarios on regtest and test them out with darksidewalletd.
I found that regtest is not stable and its time consuming to create long tests (more than 500 blocks), but its relatively simple to create a lot of transactions on some smal range of blocks and then use genblocks tools from darksidewalletd to create whatever we want to.

Describe the solution you'd like

A clear and concise description of what you want to happen.

Add an extra enum on ZcashNetwork to actually represente regtest

Alternatives you've considered

We originally thought Testnet was an alternetive but it doesn't have the flexibility and locality of Regtest

Additional context

This will allow the SDK to run tests like "Second Flush of Enthusiasm" from Zingo Labs which actually exercises a real use case of ZEC onboarding.

zingolabs/darksidewalletd-datasets#5