[webauthn] Pull Request: Add `blob` extension.

agl has just submitted a new pull request for https://github.com/w3c/webauthn:

== Add `blob` extension. ==
This allows “RPs” who, in this case will likely be browser extensions or
PWAs in offline mode, to store small amounts of arbitrary data with a
credential.

See https://github.com/w3c/webauthn/pull/1402