Serialize response context as a response header

[SergeyKanzhelev]

Same as https://github.com/aspnet/Hosting/issues/1460 for ASP.NET Core

Motivation

In distributed tracing scenarios it is not uncommon to return extra context as a response headers when possible. DiagnosticsSource Acitvity should implement API that will allow to set the response context. Fields populated by this API should be serialized as http response headers.

CC: @seth-capistron, @vancem, @jacpull, @lmolkova, @glennc, @davidfowl