Make objects implementing BuildableInterface produce a BuildableEngine

Summary:
Ref T13110. Currently, build status is published the same way for every Buildable by the BuildEngine.

I want to change this to delegate publishing to each Buildable, particularly so that Differential may use more detailed rules for handling builds and drafts.

Rather than add additional methods to the existing `BuildableInterface`, add an engine generator method instead. This is a pattern which has seen more use recently (e.g., in Ferret) and lets us pay a little more upfront to pull complex pieces of logic out of the main class and let them use inheritence more easily. If we had Traits that might cover this to some degree.

I'd expect to eventually reduce the size of `BuildableInterface` and move the `CircleCI` and `BuildKite` interfaces so that the `BuildableEngine` implements them instead of the main object.

Here, this new engine does nothing and is never instantiated. In upcoming changes, publishing logic will move into it so that Differential can handle publishing differently.

Test Plan: Ran `arc liberate`, loaded pages, grepped for `BuildableInterface`.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13110

Differential Revision: https://secure.phabricator.com/D19278