[ecash-lib] Add SigHashType

Summary: SigHashType is a class that represents all possible sighash types that can be used to sign a tx input, for both legacy (before UAHF) and BIP143 signatures.

Test Plan: npm run test && npm run build

Reviewers: bytesofman, #bitcoin_abc, PiRK

Reviewed By: bytesofman, #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15954