Added proto file and small documentation on how to regenerate the gRPC server interface

Author: Michel Vocks

Diff for: README.md

@@ -1,2 +1,6 @@
 # cppsdk
 C++ SDK for Gaia
+
+# How to generate gRPC server interfaces
+If the `plugin.proto` file has been changed, it's sometimes useful to regenerate the gRPC server interfaces.
+You can use the command `make plugin.grpc.pb.cc plugin.pb.cc` to regenerate them.

Diff for: plugin.proto

@@ -0,0 +1,59 @@
+// plugin.proto
+// Defines the gRPC interface between gaia and the user defined
+// pipelines (plugins). All rpc Methods are called from Gaia and
+// executed in the plugin. 
+
+syntax = "proto3";
+
+option java_multiple_files = true;
+option java_package = "io.gaiapipeline.proto";
+option java_outer_classname = "GRPCPlugin";
+
+package proto;
+
+// Job represents a single job
+message Job {
+    uint32              unique_id   = 1;    
+    string              title       = 2;
+    string              description = 3;
+    repeated uint32     dependson   = 4;
+    repeated Argument   args        = 5;
+    ManualInteraction   interaction = 6;
+}
+
+// Argument represents an argument passed from a pipeline 
+// to gaia and/or from gaia to the pipeline. 
+message Argument {
+    string description = 1;
+    string type        = 2;
+    string key         = 3;
+    string value       = 4;
+}
+
+// ManualInteraction represents a manual human interaction
+message ManualInteraction {
+    string description = 1;
+    string type        = 2;
+    string value       = 3;
+}
+
+// JobResult represents the result of an executed job
+message JobResult {
+    uint32 unique_id      = 1;
+    bool   failed         = 2;
+    bool   exit_pipeline  = 3;
+    string message        = 4;
+}
+
+// Empty message 
+message Empty {}
+
+service Plugin {
+    // GetJobs returns a stream of Job objects.
+    // Used to expose jobs to gaia.
+    rpc GetJobs(Empty) returns (stream Job);
+
+    // ExecuteJob signals the plugin to execute the given job.
+    // Used to execute one job from a pipeline.
+    rpc ExecuteJob(Job) returns (JobResult);
+}