VerifyScript: accumulate ScriptExecutionMetrics and return them

Summary:
This exposes the input-total metrics for VerifyScript callers, so it
can be used in CheckInputs (-> mempool tracking, scriptcache, connectblock
checking).

Test Plan: ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T704

Differential Revision: https://reviews.bitcoinabc.org/D4835