Add async overloads for ByteString.WriteTo()

[erikmav]

Related to #3166

[acozzette]

CC @jskeet @anandolee

@jtattermusch Would you be interested in reviewing this? @anandolee usually reviews C# changes but is on vacation for the next month.

[erikmav]

@acozzette Would you mind adding the "C#" tag here and in my other PR? The mergeable-bot fails on that missing label, and I can't add labels

[jtattermusch]

Same as #6230 (comment) can you describe the use case?
Also, on netstandard2.0 you can use the .Span property and write to an output stream (or to a pipewriter) in a more flexible fashion.

[erikmav]

Good point and I'll move toward that. In this case I'm working in a net472 codebase, the .Span property is not visible in this case, so I was optimizing for what was available to minimize thread waits.

[erikmav]

@jtattermusch Memory only supported on netcore2.1+. I'd prefer to get this change in for downlevel net472 optimization.

[jtattermusch]

Same response as on #6230 (comment)

[erikmav]

@jtattermusch This is different from #6230 as it simply adds the ability to write to a stream without holding a thread context. I closed that PR, but this one still seems useful by itself.

[jtattermusch]

Once we add the System.Memory dependency for all targets (see #6317), it seems that this will no longer be useful, because you can grab byteString.Span and use that to write to the Stream? If that's the case, let's close because then there wouldn't be much point in adding new APIs.