Adding key management interface

Exposes local keying material and a control for it.

This doesn't do anything about presenting a view of the remote party keys.

For https://www.w3.org/Bugs/Public/show_bug.cgi?id=21880
The feedback we got from the web cryptography files was that we were better off defining this and owning it. They were reluctant to take on an effectively useless key usage.