[CBOR] Implement a proof of concept for ECDsa COSE key serialization

[eiriktsarpalis]

Implements a proof-of-concept serializer for ECDsa COSE key serialization, as defined in https://www.w3.org/TR/webauthn/#sctn-encoded-credPubKey-examples.

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Tagging subscribers to this area: @bartonjs, @vcsjones, @krwq
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[bartonjs]

Apparently my Approve (with feedback) from a couple days ago didn't post.

This box said something like "LGTM, but a few comments to make it more sample-friendly".

[bartonjs]

I was curious to see if you were going to take advantage of this in the reader.

For a public method I'd expect the conformance mode check to be a runtime check => ArgumentException (not just an assert). For internal an assert would make sense, since the only callers should have either validated it themselves or were the ones who created the reader.

To make this Proof of Concept more sample-friendly, I'd switch the access modifier to internal.

[eiriktsarpalis]

I was curious to see if you were going to take advantage of this in the reader.

If you mean the field ordering guarantee, I think it's mostly effective in scenaria where you don't support optional fields. In the general case something like this might be preferable.

[bartonjs]

Oh, to be clear, I approve of taking the dependency; it's part of the niceness of the conformance mode 😄.

[eiriktsarpalis]

Sure, I suppose my point is that even assuming CTAP2 conformance mode, you might still need to opt for something like the original approach if you needed to tolerate arbitrary fields (e.g. because of forward compatibility concerns).

[krwq]

LGTM after fixing comments