reporting: restructure RRD handling for health graphs

[fichtner]

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Is your feature request related to a problem? Please describe.

rrd.inc is an ancient integration and should likely be removed (and for the most part it already was on the development version).

Describe the solution you like

Replace with flexible drop-in scripts and common boilerplate handling magic.

Describe alternatives you considered

N/A.

Additional context

N/A

[AdSchellevis]

implemented in #7836