Add a basic "harbormaster.step.edit" API method

Summary: Ref T13585. Provide a minimal but technically functional "harbormaster.step.edit" API method.

Test Plan: Used the web console to modify the URI for a "Make HTTP Request" build step.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13585

Differential Revision: https://secure.phabricator.com/D21489