Implement execution create api

protobuf/api/execution.proto

@@ -11,6 +11,9 @@ package api;
 // The source file of this API is hosted on [GitHub](https://github.com/mesg-foundation/engine/blob/master/protobuf/api/execution.proto).
 
 service Execution {
+  // Create creates a single Execution specified in a request.
+  rpc Create(CreateExecutionRequest) returns (CreateExecutionResponse) {}
+
   // Get returns a single Execution specified in a request.
   rpc Get(GetExecutionRequest) returns (types.Execution) {}
 
@@ -22,6 +25,20 @@ service Execution {
   rpc Update(UpdateExecutionRequest) returns (UpdateExecutionResponse) {}
 }
 
+// CreateExecutionRequest defines request to create a single execution.
+message CreateExecutionRequest {
+  string instanceHash = 1;
+  string taskKey = 2;
+  string inputs = 3;
+  repeated string tags = 4;
+}
+
+// CreateExecutionResponse defines response for execution creation.
+message CreateExecutionResponse {
+  // Execution's hash.
+  string hash = 1;
+}
+
 // GetExecutionRequest defines request to retrieve a single execution.
 message GetExecutionRequest {
   // Execution's hash to fetch.

server/grpc/api/execution.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 
 	"github.com/mesg-foundation/core/execution"
 	"github.com/mesg-foundation/core/hash"
@@ -27,6 +28,29 @@ func NewExecutionServer(sdk *sdk.SDK) *ExecutionServer {
 	return &ExecutionServer{sdk: sdk}
 }
 
+// Create creates an execution.
+func (s *ExecutionServer) Create(ctx context.Context, req *api.CreateExecutionRequest) (*api.CreateExecutionResponse, error) {
+	hash, err := hash.Decode(req.InstanceHash)
+	if err != nil {
+		return nil, err
+	}
+
+	var inputs map[string]interface{}
+	if err := json.Unmarshal([]byte(req.Inputs), &inputs); err != nil {
+		return nil, fmt.Errorf("cannot parse execution's inputs (JSON format): %s", err)
+	}
+
+	executionHash, err := s.sdk.Execution.Execute(hash, req.TaskKey, inputs, req.Tags)
+	if err != nil {
+		return nil, err
+	}
+
+	return &api.CreateExecutionResponse{
+		Hash: executionHash.String(),
+	}, nil
+
+}
+
 // Get returns execution from given hash.
 func (s *ExecutionServer) Get(ctx context.Context, req *api.GetExecutionRequest) (*types.Execution, error) {
 	hash, err := hash.Decode(req.Hash)