GRPC integration tests

[eguzki]

Add integration tests with e2e GRPC calls.

Add required changes (maybe in https://github.com/3scale/Test-APIcast) to have some preprocessor like --- grpc_request and same for responses

Library that might be used, which needs to be investigated: https://github.com/jinq0123/grpc-lua

[tkan145]

Perhaps we can include grpcurl with the image https://github.com/fullstorydev/grpcurl? Then add a preprocessor like --- grpc_request to use gprcurl, similar to how openresty test-nginx uses curl