Use an HTTPEngineExtension to implement "https.blindly-trust-domains" in Arcanist

Summary: Ref T10227. This converts weird hard-codey magic to the new HTTPEngineExtension.

Test Plan: See D16090.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10227

Differential Revision: https://secure.phabricator.com/D16091