feat: add support for aliases

v2/pkg/engine/datasource/grpc_datasource/compiler.go

@@ -323,7 +323,14 @@ func (p *RPCCompiler) Compile(executionPlan *RPCExecutionPlan, inputData gjson.R
 
 	for _, call := range executionPlan.Calls {
 		inputMessage := p.doc.MessageByName(call.Request.Name)
+		if inputMessage.Name == "" {
+			return nil, fmt.Errorf("input message %s not found in document", call.Request.Name)
+		}
+
 		outputMessage := p.doc.MessageByName(call.Response.Name)
+		if outputMessage.Name == "" {
+			return nil, fmt.Errorf("output message %s not found in document", call.Response.Name)
+		}
 
 		request := p.buildProtoMessage(inputMessage, &call.Request, inputData)
 		response := p.newEmptyMessage(outputMessage)

v2/pkg/engine/datasource/grpc_datasource/execution_plan.go

@@ -166,6 +166,15 @@ type RPCField struct {
 	Message *RPCMessage
 }
 
+// AliasOrPath returns the alias of the field if it exists, otherwise it returns the JSONPath.
+func (r *RPCField) AliasOrPath() string {
+	if r.Alias != "" {
+		return r.Alias
+	}
+
+	return r.JSONPath
+}
+
 // RPCFields is a list of RPCFields that provides helper methods
 // for working with collections of fields.
 type RPCFields []RPCField