Allow franka_gripper messsage and action definitions to be built without libfranka dependency.

[smihael]

Currently, the franka_gripper package contains both the message and action definitions as well as control nodes, depending on the libfranka library, which is unnecessary for users who only need to send commands to an already existing gripper control node. I suggest either moving the message and action definitions to the franka_msgs package (which would break code compatibility) or introducing a build flag to allow building franka_gripper without requiring libfranka. This would enable users to create lighter workspaces that only contains the necessary message structures and actions to communicate with the robot.