Allow getting the corresponding application for config options master
Summary: Introduce `getApplicationClassName()` in PhabricatorApplicationConfigOptions returning null, overwritten in child classes to return the corresponding application class when providing config options specific to some application. Note that PhabricatorDiffusionConfigOptions and PhabricatorRepositoryConfigOptions both return Diffusion, so duplicate values are possible. Refs T16351 Test Plan: None; it's an additional function to be used by future code. Reviewers: O1 Blessed Committers, avivey Reviewed By: O1 Blessed Committers, avivey Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T16351 Differential Revision: https://we.phorge.it/D26526