Provide API to reduce string allocations in user-defined custom converters

[AArnott]

See the CommonString struct defined in vs-streamjsonrpc (like this one) which currently uses MessagePack-CSharp APIs. We should expose equivalent APIs from this library, and/or expose a CommonString-like struct so that the user doesn't have to write it.

[AArnott]

CC: @trippwill

[trippwill]

In adding support for this to vs-streamjsonrpc, we have a many instances of this pattern for CommonString like Version2, which would be awesome to simplify:

        if (!reader.TryReadStringSpan(out ReadOnlySpan<byte> valueBytes))
        {
            // TODO: More specific exception type
            throw new NBMP::MessagePackSerializationException(Resources.UnexpectedErrorProcessingJsonRpc);
        }

        // Recognize "2.0" since we expect it and can avoid decoding and allocating a new string for it.
        if (Version2.TryRead(valueBytes))
        {
            return Version2.Value;
        }

[AArnott]

Am I reading this right? It looks bad:

  if (!reader.TryReadStringSpan(out ReadOnlySpan<byte> valueBytes))
{
  // TODO: More specific exception type
  throw new NBMP::MessagePackSerializationException(Resources.UnexpectedErrorProcessingJsonRpc);
}

TryReadStringSpan is supposed to be able to return false when the string isn't contiguous in memory, so throwing in a perfectly legitimate (though uncommon) case seems broken.

[trippwill]

This is the code path from MessagePackFormatter. If we can't get the string as a span of bytes, there's nowhere to go:

        ReadOnlySpan<byte> valueBytes = MessagePack.Internal.CodeGenHelpers.ReadStringSpan(ref reader);
        if (Version2.TryRead(valueBytes))
        {
            return Version2.Value;
        }

[AArnott]

This is the code path from MessagePackFormatter

That one is fine, because if the string isn't contiguous, it'll allocate memory and make it contiguous. It's the if (!TryReadStringSpan) that's buggy.
But with the new API I'm developing in NB.MP, hopefully that code will go away anyway in favor of something simpler. So no worries about it for now.