Remap the diagnostic output of the oakd node when using a namespace

[civerachb-cpr]

Description

When using a namespace the oakd node still publishes diagnostics to /diagnostics instead of e.g. /donatello/diagnostics.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Tested on cpr-april by manually applying changes to launch file in /opt and restarting turtlebot4.service.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation