Add skeleton chain and client classes

Summary:

This commit does not change behavior. It just adds new skeleton classes
that don't do anything and aren't instantiated yet.

Partial backport of core PR14437:
https://github.com/bitcoin/bitcoin/pull/14437/commits/7e2e62cf7c513bd7d8e784069c5534fda1c50c52

Depends on D5058.

Test Plan:

ninja check
make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5064