Add an Almanac service type for Drydock to lease against

Summary: Ref T9253. See D13843 for some discussion. This is very bare-bones for now since I believe that almost all interesting configuration (e.g., credentials) should live in Drydock, although I imagine it getting some configuration eventually.

Test Plan: Used Almanac → Services → Create Service to create a new service of this type.

Reviewers: hach-que, chad

Reviewed By: hach-que, chad

Maniphest Tasks: T9253

Differential Revision: https://secure.phabricator.com/D14109