Runtime: API Proposal: Make IncrementalHash.CreateHMAC ReadOnlySpan<byte> key public

[vcsjones]

Since HKDF was added, IncrementalHash has had support for ReadOnlySpan<byte> HMAC keys, but it isn't made public.

The functionality is already there, it just needs to be changed from internal to public and given proper test coverage.

Proposal:

 namespace System.Security.Cryptography {
     public sealed class IncrementalHash : IDisposable {
        // existing method changed from internal to public
        public static IncrementalHash CreateHMAC(HashAlgorithmName hashAlgorithm, ReadOnlySpan<byte> key);
     }
 }

/cc @JimBobSquarePants

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[vcsjones]

@bartonjs @carlossanlop why is this no longer ready for review?

[bartonjs]

@vcsjones Bulk operation error, fixed now.

[terrajobst]

Video

Looks good as proposed.

```C#
namespace System.Security.Cryptography
{
public partial class IncrementalHash
{
public static IncrementalHash CreateHMAC(HashAlgorithmName hashAlgorithm, ReadOnlySpan key);
}
}