Add support for credProps extension

[iinuwa]

Since the credProps extension is useful for relying parties to design better UI flows, 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 credProps extension as defined in the spec.

[iinuwa]

This one may be missing from #15. Need to confirm

[msirringhaus]

Should be there in libwebauthn at least: https://github.com/linux-credentials/libwebauthn/blob/master/libwebauthn/src/proto/ctap2/model/make_credential.rs#L355-L383

I thought it worked, but I have to double check.

[iinuwa]

Completed in #31.