Import draft BUIP-HF spec and replay-protected-sighash spec

Summary:
Specification documents for BUIP-HF.

These are both still under review and subject to PRs on BUIP repo, so
further updates will be highly likely.

These documents should be kept in sync with the BUIP repository versions.

Test Plan:

There is a test plan inside the tech spec which should be extracted to
a separate document in future.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D133