This commit effectively moves the definition of these constants
out of the chainparamsbase to their own file.
Using the ChainType enums provides better type safety compared to
passing around strings.
The commit is part of an ongoing effort to decouple the libbitcoinkernel
library from the ArgsManager and other functionality that should not be
part of the kernel library.
Backport note: the Config ctor could also take a ChainType param instead of a network string but lets do this in a separate diff
This is a partial backport of core#27491
https://github.com/bitcoin/bitcoin/pull/27491/commits/ba8fc7d788932b25864fb260ca14983aa2398c23
Depends on D17987