[ecash-lib] Add pushBytesOp to make a minimal bytes pushop

Summary:
Script requires to use minimally encoded push ops. pushBytesOp creates a minimal Op for us that pushes the given bytes.

Depends on D15938.

Test Plan: npm run test && npm run build

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15940