Add grpc-context support

[sjoerdtalsma]

Is your feature request related to a problem? Please describe.

Grpc Context offers a fluent api (starting from the static Context class).
Let's see if it could be fully supported as well from this context-propagation library and how much effort it would cost.

Describe the solution you'd like

Clean grpc-context propagation without hacks or workarounds using a delegating ContextManager.

Describe alternatives you've considered

none, I just realised this was not even on the radar yet due to lots of other work

Additional relevant information