Allow users to provide additional labels for the NGINX Gateway Fabric control plane service

[michasHL]

Is your enhancement request related to a problem? Please describe.
As a user I want to provide additional service labels to my NGINX Gateway Fabric control plane service.

What would you like to be added:
I would like to specify additional labels for my NGINX Gateway Fabric control plane service.

Why this is needed:
Currently a user is not able to specify additional service labels. The helm chart adds some internal labels but not does not allow users to provide their own.
We're using prometheus service monitors that use service labels to make sure the metrics from the service can be ingested.

[nginx-bot]

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