Refactor signature encoding checks

Summary:
Isolate ECDSA-specific parts in preparation for Schnorr.

Next steps would be:

• Disallow 64 byte ECDSA signatures in CheckRawECDSASignatureEncoding when SCRIPT_ENABLE_SCHNORR flag is set (D2469)
• Add Schnorr-specific functions for checking signature encodings

Test Plan: make check

Reviewers: markblundeberg, deadalnix, #bitcoin_abc

Reviewed By: markblundeberg, deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2477