[alias-server] push parsing for op return

Summary:
T3060

Use the consume function to build a feature complete push-parsing function for OP_RETURN.

OOM vector of large pushes is not directly handled. An error will be thrown though if pushdata exceeds the length of the provided string.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13964