Feature: Include public key in certificate

[joshlang]

See dotnet/runtime#33744

While it looks like it will be fixed, it seems that by including the public key in the certificate, it might save some headache (and computation). Without it, the public key needs to be derived from the private key, which is causing trouble on platforms that don't have fully-featured support.

Our code is essentially:

var priv = KeyFactory.NewKey(KeyAlgorithm.ES256);
var generated = order.Generate(..., priv);
var pfxBuilder = generated.ToPfx(priv);
var cert = pfxBuilder.Build(host, password);

[stale]

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