R1:e64efa05c3ff

Implement "Run Remote Command" build step implementation.

Summary: This adds a build step implementation for running a command on a remote machine over SSH. It supports merging in various variables about the build (such as the commit hash / revision ID, repository call sign, version control type and clone URI).

Test Plan: Configured a build plan to run `/bin/true` on localhost and the build passed. Configured a build plan to run `/bin/false` on localhost and the build failed.

Reviewers: epriestley…