Add support for build trigger payloads

Summary:
Payloads to use when triggering a build to select what
exactly should be built.

Documentation is sparse, so this is based on examples
given at the /~/settings/automated-builds/ repo page
on Docker Hub.

Ref T965

Test Plan: Run tests, fire build with JSON encoded version of these payloads.

Reviewers: dereckson

Tags: TDD

Maniphest Tasks: T965

Differential Revision: https://devcentral.nasqueron.org/D596