[webauthn] How to get supported client extension (#1199)

Kieun has just created a new issue for https://github.com/w3c/webauthn:

== How to get supported client extension ==
Currently, the WebAuthn does not define the way for RP to get supported extension that the client supports.
For the authenticator extension support, there is a _exts_ extension so that RP may obtain such information during the create() call.
Similarly, the RP would like to know which extensions are supported by user's client.



Please view or discuss this issue at https://github.com/w3c/webauthn/issues/1199 using your GitHub account