Add application/grpc media type

[mbore]

required by softwaremill/tapir#2345

[adamw]

Sorry, binary incompatible ... we'll have to come up with a better encoding of enumerating media types so that we can evolve the interface.

But you should be able to define a custom media type without being it present in sttp-model?

[mbore]

Yes, I am. I just wanted to add it to this model since it's a common media type, but will add it in tapir.