Give ConduitAPIMethod->getMethodDescription() access to a real Viewer

Summary:
Ref T9964. The new *.search and *.edit methods generate documentation which depends on the viewer.

For example, the *.search methods show a reference table of the keys for all your saved queries.

Give them a real viewer to work with.

During normal execution, just populate this viewer with the request's viewer, so $request->getViewer() and $this->getViewer() both work and mean the same thing.

Test Plan: {F1023780}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14761