Added a script to generate chainparams constants from intermediate files

Summary:
Building on D3732 so that chainparams constants no longer need to be copy-pasted manually.

Depends on D4140

Completes T663

Test Plan:
For actually testing the process flow, generate the chainparams_*.txt files:

bitcoind
make_chainparams.py > chainparams_main.txt

bitcoind --testnet
make_chainparams.py -a 127.0.0.1:18332 > chainparams_main.txt

For this diff, I did not do the above, I copied the existing chainparams manually into the chainparams_*.txt files so that we don't add updating those to the review process of this diff.
After the chainparams_*.txt files are ready, do the following:

generate_chainparams_constants.py .

make check
ninja check

TeamCity IBD

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4120