Add documentation on verifying app attest responses

[lol768]

@aseigler Is there any documentation covering how to use the support introduced in #322?

I appreciate the test is there, but AppleAppAttest is an internal class, as far as I can see, so none of the logic in the test classes is useful to someone using the library.

All of the other examples seem very web-oriented, for example I just have a Base64 blob from my iOS app - I have no idea what I'd pass to AssertionOptions.FromJson

[lol768]

Also, I guess before trying to use this support, I need a release/NuGet package that ships it

[dbeinder]

Please put trustPath and attestationType back into AttestationVerificationSuccess, and maybe even the entry of the metadata service.
Before the 4.0 refactoring, the attestation chain was available.

@lol768 for now, I access the attestation chain like this:

var attestation = AuthenticatorAttestationResponse.Parse(rawResponse).AttestationObject;
var verifier = AttestationVerifier.Create(attestation.Fmt);
var clientDataHash = SHA256.HashData(rawResponse.Response.ClientDataJson);
(var attType, var trustPath) = verifier.Verify(attestation.AttStmt, attestation.AuthData, clientDataHash);
Console.WriteLine($"Attestation Type: {attType}");
if (trustPath != null)
{
    for (var cn = 0; cn < trustPath.Length; cn++)
    {
        var cert = trustPath[cn];
        Console.WriteLine($"Attestation[{cn}] Subject: {cert?.Subject}");
        Console.WriteLine($"               Issuer:  {cert?.Issuer}");
        Console.WriteLine($"               Serial:  {cert?.SerialNumber}");
        Console.WriteLine($"               Valid:   {cert?.NotBefore:yyyy-MM-dd} - {cert?.NotAfter:yyyy-MM-dd}");
    }
}