HTTP/2 benchmarks

Create a suite of benchmarks at https://github.com/aspnet/benchmarks to cover most common user scenarios. Can reuse profiling/investigation samples - https://github.com/aspnet/AspNetCore/issues/13745

* [ ] TechEmpower plaintext but over HTTP/2
* [ ] Basic POST. Request is text/plain string of name. Reply is text/plain `"Hello " + name`
* [ ] Stream per connection vs multiple streams on one connection
* [ ] H2 vs H2C
* [x] gRPC benchmarks. Will be created by gRPC team - https://github.com/grpc/grpc-dotnet/issues/6
* Unary
* Server streaming
* Large messages
* [ ] Kestrel and IIS HTTP/2 benchmarks

Some of these benchmarks may already exist.