Added a mock convert query service

Implements the ConvertQuery method and just returns a hardcoded string
value.

internal/usecases/convertquery/mock/mockconvertquery.go

+package mockconvertquery
+
+// A Service implements the query convert usecase interface (mock)
+type Service struct {
+}
+
+// NewService creates a new query convert service (mock)
+func NewService() *Service {
+	return &Service{}
+}
+
+// ConvertQuery returns a hard coded string message (mock)
+func (s *Service) ConvertQuery(jsonMsg *[]byte) (*string, error) {
+	mockQuery := "Query converted"
+	return &mockQuery, nil
+}