[chromedriver] Add AddVirtualAuthenticator command.

Add the `AddVirtualAuthenticator` command to ChromeDriver.

A draft of the proposed addition to the specification is available at
https://github.com/nsatragno/webauthn/compare/master...nsatragno:automation

This is one in a series of patches intended to create a Testing API for
WebAuthn, for use in Web Platform Tests and by external webauthn tests.

For an overview of overall design, please see
https://docs.google.com/document/d/1bp2cMgjm2HSpvL9-WsJoIQMsBi1oKGQY6CvWD-9WmIQ/edit?usp=sharing

Bug: 922572
Change-Id: Ia78e89641a5c0b7c1e3df46dc212fedffa8fc372
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1689037
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677807}