Write unary content with single Stream.WriteAsync

[JamesNK]

Large performance improvement in unary RPS.

dotnet/runtime#35184 (comment)

Before

Successfully processed 100373; RPS 27299; Errors 0; Last elapsed 3.9209ms
Successfully processed 127896; RPS 27496; Errors 0; Last elapsed 3.6429ms
Successfully processed 155281; RPS 27374; Errors 0; Last elapsed 3.6593ms
Successfully processed 182801; RPS 27520; Errors 0; Last elapsed 3.546ms
Successfully processed 209917; RPS 27112; Errors 0; Last elapsed 3.6468ms

After

Successfully processed 228755; RPS 52340; Errors 0; Last elapsed 2.1993ms
Successfully processed 282848; RPS 53993; Errors 0; Last elapsed 2.1865ms
Successfully processed 336652; RPS 53802; Errors 0; Last elapsed 1.5827ms
Successfully processed 390310; RPS 53628; Errors 0; Last elapsed 0.9105ms
Successfully processed 443199; RPS 52889; Errors 0; Last elapsed 2.2438ms

When Google.Protobuf supports writing to IBufferWriter<byte> this can be improved to write message data directly to the buffer array.

@stephentoub

[JunTaoLuo]

Impressive improvements!

[maxbl4]

Do I understand correctly: these improvement can only be seen on net 5.0 runtime?
I ran some tests with grpc.net 2.27 on 3.1.300, grpc.net 2.29 on 3.1.300 and
grpc.net 2.29 on 5.0.100-preview.7.20304.1-win-x64
and see perf improvement only on net 5.0