duplicate API CheckTransactionSignatureEncoding -> CheckTransactionECDSASignatureEncoding

Summary:
... the latter for being used in OP_CHECKMULTISIG. Now each of
CHECKDATASIG/CHECKSIG/CHECKMULTISIG calls its own custom encoding checker.

Rationale: We are switching to a different strategy where 64-byte ECDSA
signatures will now also be banned from OP_CHECKMULTISIG once
SCRIPT_ENABLE_SCHNORR activates. (schnorr spec has not yet been updated
on this point). In a child Diff, that behaviour will be added.

Test Plan:
behaviour is only duplicated; tests in sigencoding_tests have
been duplicated in accordance.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2466