Add Task-based async API for Socket.SendFile

[geoffkizer]

Background and Motivation

We have an APM version of this and a SocketAsyncEventArgs version, but no Task-based version.

Current APM:

    public IAsyncResult BeginSendFile(string fileName, AsyncCallback? callback, object? state);
    public IAsyncResult BeginSendFile(string? fileName, byte[]? preBuffer, byte[]? postBuffer, TransmitFileOptions flags, AsyncCallback? callback, object? state); 

Proposed API

    public ValueTask SendFileAsync(string fileName);
    public ValueTask SendFileAsync(string? fileName, byte[]? preBuffer, byte[]? postBuffer, TransmitFileOptions flags);

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[terrajobst]

Video

• Makes sense, but we should take a cancellation tokena and memories

namespace System.Net.Sockets
{
    public partial class Socket
    {
        // Existing APM APIs
        // public IAsyncResult BeginSendFile(string fileName, AsyncCallback? callback, object? state);
        // public IAsyncResult BeginSendFile(string? fileName, byte[]? preBuffer, byte[]? postBuffer, TransmitFileOptions flags, AsyncCallback? callback, object? state);
        public ValueTask SendFileAsync(string? fileName, CancellationToken cancellationToken = default);
        public ValueTask SendFileAsync(string? fileName, ReadOnlyMemory<byte> preBuffer, ReadOnlyMemory<byte> postBuffer, TransmitFileOptions flags, CancellationToken cancellationToken = default);
    }
}

[gfoidl]

@stephentoub can you assign this to me?
(Will do a PR in the next couple of days)