[API Proposal]: Make GenericNameAsn (and related) available via X509SubjectAlternativeNameExtension

Start with src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/X509SubjectAlternativeNameExtension.cs and review the existing EnumerateDnsNames() and EnumerateIPAddresses() APIs. Compare the proposed aggregate and per-type enumeration designs with the related internal ASN classes; done requires a settled public API design rather than simply exposing the currently internal types.

Background and motivation

I'm the maintainer of a small ACME compatible server software, that connects ACME to ADCS.
Currently the implementation uses CERTENROLLLib to validate data around CSRs.
I now tried to move that CSR validation into managed code, to have only small code snippets that need COM interop at all and else have a generic ACME server, that might support multiple backends.

While src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/X509SubjectAlternativeNameExtension.cs would contain lots of the code, that is necessary to read CSRs (and especially the SANs), all besides DnsNames and IPAddresses is marked internal, which leads to duplicating code from some of those classes into my own project.

API Proposal

namespace System.Security.Cryptography.X509Certificates
{
    public sealed class X509SubjectAlternativeNameExtension : X509Extension
    {
        // existing API omitted
        
        public IEnumerable<GeneralNameAsn> EnumerateAlternativeNames();
    }
}

API Usage

var certificateRequest = CertificateRequest.LoadSigningRequest(
    Convert.FromBase64String(order.CertificateSigningRequest),
    HashAlgorithmName.SHA256,
    CertificateRequestLoadOptions.UnsafeLoadCertificateExtensions 
    );

var alternativeNames = certificateRequset.CertificateExtensions.OfType<X509SubjectAlternativeNameExtension>().SelectMany(x => x.AlternativeNames);

Alternative Designs

Since there are already methods for DnsNames and IPAddresses EnumerateDnsNames(), EnumerateIPAddresses()
There could be enumeration by type

namespace System.Security.Cryptography.X509Certificates
{
    public sealed class X509SubjectAlternativeNameExtension : X509Extension
    {
        // existing API omitted
        
        public IEnumerable<OtherNameAsn> EnumerateOtherNames();
        public IEnumerable<string> EnumerateRfc822Names();
        public IEnumerable<byte[]> EnumerateX400Addresses();
        public IEnumerable<byte[]> EnumerateDirectoryNames();
        public IEnumerable<EdiPartyNameAsn> EnumerateEdiPartyNames();
        public IEnumerable<string> EnumerateUris();
        public IEnumerable<string> EnumerateRegisteredIds();
    }
}

Risks

This entails making a bunch of classes public, that currently are internal