Add a flag to activate OP_CHECKDATASIG

Summary:
As per title.

Depends on D1563

Test Plan: Add test cases in script_test.json and checkdatasig_tests.cpp

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1564