R1:29436dfe378c

Added 'method' field to the HTTP request build step.

Summary:
This revision adds a 'method' field to the HTTP request harbormaster build step. This allows the user to specify GET, POST, DELETE, and PUT (limited by the underlying wrapper phabricator uses for HTTP requests). I'm not sure how much sense PUT makes, but oh well.

Existing plans shouldn't break, as if this field is an empty string, we default to POST, which is the old behavior.

Fixes T4604

Test Plan: 1) Verified that the empty string does…