feat: add focus and iris service

[andreacalm]

Add focus and iris control services to PTZ node

What changed

• axis_control.py: extended sendPTZCommand() with optional focus, autofocus, iris, autoiris params
• axis_ptz_node.py: added ~set_focus and ~set_iris services, internal desired state for optics, restored ~camera_params publisher
• srv/set_focus.srv, srv/set_iris.srv: new service definitions
• CMakeLists.txt: registered new services
• README.md: documented new services with usage examples

Behaviour

• autofocus=true → sends autofocus=on, camera controls focus automatically
• autofocus=false + focus=<val> → sends manual focus value
• Same logic for ~set_iris / autoiris
• If camera does not support focus/iris: rospy.logwarn, no fatal exception
• Value in ~camera_params reflects what the camera actually applied

Not changed
~ptz_command, ~home_ptz and all robotnik_msgs interfaces untouched.