Templatize ValidationState instead of subclassing

Summary:

This removes boilerplate code in the subclasses which otherwise only
differ by the result type.

Completes backport 3/3 of core PR17399:
https://github.com/bitcoin/bitcoin/pull/17399/commits/10efc0487c442bccb0e4a9ac29452af1592a3cf2

Depends on D8327.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8328