A rebase of kallewoof/bitcoin#5 on top of #432 taking the opportunity to simplify its changes around chainparams code and adding a test that also acts as a script to generate a private key in wallet importable format and its corresponding p2pk blockscript, which should be useful for people wanting to generate their own signets.
This represent a hardfork from that implementation because the genesis block now includes the name of the chain too, like all other custom chains, and the bits have to be low enough so that tests keep working with regtest-like pow.

Example to generate chain arguments:

python3 ./test/functional/test_runner.py feature_signet --config_path=/home/user/signet

That will generate:

/home/user/signet/signet_blockscript.txt
/home/user/signet/wif.txt

Dependencies:

• [0.17] Test: Fix example_test.py [0.17] Test: Fix example_test.py #434
• [0.17] QA: Use resgtest2 chain instead of regtest for rpc tests [0.17] QA: Use resgtest2 chain instead of regtest for rpc tests #432
• [0.17] Chainparams: Introduce consensus.genesis_style for compatibility [0.17] Chainparams: Introduce consensus.genesis_style for compatibility #436