Support PRF extension

[iinuwa]

Since the PRF extension is useful—for example, for password managers wanting to securely unlock vaults—it would be good for us to support so that clients don't have to replicate this functionality.

Once #5 is completed, we should add support to process the PRF extension as defined in the spec.

[iinuwa]

This may have been resolved in #15, but needs to be verified. I don't currently have a device that can test this.

[msirringhaus]

It works for GetAssertion (where the PRF actually gets calculated), but we seem to be missing a required response field when doing MakeCredential. I have to double-check that. It used to work.