Added a script to generate chainparams intermediate files

Summary:
First step to generating chainparams header files. Next step is to
generate the header file based on the output of running this script
once for each chain.

In the near future, this pipeline becomes possible (all TODO):

• A bot with up-to-date nodes runs this script.
• Chainparams header is generated.
• Smoke tests are run on the result and landed on master automatically.

For now, this script can be used manually.

Split off from D3502. This iteration achieves a simpler design by being split
into two steps.

Progress towards T663

Test Plan:

make check-devtools
ninja check-devtools

# show help message
make_chainparams.py --help

# errors due to missing config/config params
make_chainparams.py -c blah
make_chainparams.py
vim ~/.bitcoin/bitcoin.conf # add rpcuser
make_chainparams.py
vim ~/.bitcoin/bitcoin.conf # add rpcpassword

# generates mainnet output
bitcoind
make_chainparams.py

# generates testnet output
bitcoind --testnet
make_chainparams.py -a "127.0.0.1:18332"

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3732