Add a "Make an HTTP Request" build step

Summary:
Ref T1049. This is very minimal, but does what it says.

I merged the variable replacement code so Remote + HTTP can share more stuff.

Test Plan:
Ran "HTTP" and "Remote" build plans.

{F79886}

{F79887}

Reviewers: hach-que, btrahan

Reviewed By: hach-que

CC: zeeg, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7541