[Extensions] Implement Extension trait

[msirringhaus]

I think it may make sense to define a traits for client extension processing, where each extension implements the trait including at least two methods - equivalent to create() and get() from the CTAP 2.2 spec, for processing on MakeCredential and GetAssertion respectively. In turn, this could be generic over the respective extension's client extension input, eg.

12.2.1. Client extension input

create() : ArrayBuffer containing opaque data in an RP-specific format.

partial dictionary AuthenticationExtensionsClientInputs {
  ArrayBuffer credBlob;
};

get() : A boolean value to indicate that this extension is requested by the Relying Party.

partial dictionary AuthenticationExtensionsClientInputs {
  boolean getCredBlob;
};

Originally posted by @AlfioEmanueleFresta in #77 (comment)