Add dependency on ctx in app file

[janchochol]

Otherwise, it is not included in release of applications dependeing on
grpcbox.

[fenollp]

From the code (the way generated code checks whether an argument is #ctx I think this was on purpose, leaving the choice of context implementation to the user.

But I agree this should at least be added to the interop example app in this repo, just so that it's easier for newcomers to start using grpcbox ;)

[tsloughter]

Yea, I'll merge this. I have alternate plans for ctx going forward and that argument will likely be removed and the process dictionary used instead.

Once I separate out the context from the opentelemetry-erlang tracer open-telemetry/oteps#42

[janchochol]

I do not think, that using ctx is somehow optional - e.g. grpcbox_client_stream:metadata_headers/1 does expect ctx module.