[Extensions] Implement Extension trait

Nobody has claimed this yet.

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](https://fidoalliance.org/specs/fido-v2.2-rd-20230321/fido-client-to-authenticator-protocol-v2.2-rd-20230321.html#credBlobFeatureDetection)
> > [create()](https://w3c.github.io/webappsec-credential-management/#dom-credentialscontainer-create) : ArrayBuffer containing opaque data in an RP-specific format.
> > ```javascript
> > partial dictionary AuthenticationExtensionsClientInputs {
> > ArrayBuffer credBlob;
> > };
> > ```
> > [get()](https://w3c.github.io/webappsec-credential-management/#dom-credentialscontainer-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 https://github.com/linux-credentials/libwebauthn/issues/77#issuecomment-2674105649_

No files or tests are identified; start by locating the existing extension handling for MakeCredential and GetAssertion, then compare it with the CTAP 2.2 create() and get() extension operations. Done means defining an Extension trait with those operations and generic client extension inputs, with the integration scope and behavior resolved.

Written by the indexing model from the issue text.