Add Span overloads for Socket.SendFile

[geoffkizer]

Proposed API

namespace System.Net.Sockets
{
    public class Socket
    {
        // Existing API
        // public void SendFile(string fileName, byte[] preBuffer, byte[] postBuffer, System.Net.Sockets.TransmitFileOptions flags);

        public void SendFile(string fileName, ReadOnlySpan<byte> preBuffer, ReadOnlySpan<byte> postBuffer, System.Net.Sockets.TransmitFileOptions flags);
    }
}

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[geoffkizer]

Related (async APIs): #42591

[gfoidl]

I can do this together with #42591 when this api is approved.
(need to finish something for work, then I'll complete the change for the async PR)

[terrajobst]

Video

• Looks good as proposed
• We considered span-ifying the string parameter, but that feels contrived

namespace System.Net.Sockets
{
    public class Socket
    {
        // Existing API
        // public void SendFile(string fileName, byte[] preBuffer, byte[] postBuffer, TransmitFileOptions flags);

        public void SendFile(string fileName, ReadOnlySpan<byte> preBuffer, ReadOnlySpan<byte> postBuffer, TransmitFileOptions flags);
    }
}