Generate assumed blockchain and chainstate disk sizes when updating chainparams

Summary:
These values are currently updated by hand (rarely and inconsistently). This patch automates
this process so we no longer have to.

Test Plan:

• ninja check-devtools
• Execute steps in contrib/devtools/chainparams/README.md and verify they generate similar output
• Run IBD in CI

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6289