[core] Rework variables handling

[ricoberger]

This commit changes the variables handling, like it was discussed in
issue #250. This means that each plugin can now export a "variables"
function to load variables in a dashboard. The only exception is the
"core" plugin, where the variables are still handled within the
Dashboards component.

This change allows us, to introduced variables for more plugins, like a
list of resource groups via the Azure plugin or a list of field values
via the klogs plugin.

We also adjusted the naming and location of some interfaces for this
change. So that the interfaces for the CRDs are now prefixed with the
name of the CRD (e.g. "IPlaceholder" becomes "IDashboardPlaceholder").
This was necessary to reduce conflicts in names and to avoid cycle
imports.

The documentation for the variables options which can be used in a
dashboard, should be placed on the corresponding plugin page and a link
should be added to the "Variable Plugin Options" section in the
dashboards documentation.

Closes #250.

• I added a CHANGELOG entry for this change.
• I adjusted the corresponding documentation for this change.
• I adjusted the values.yaml file and the documentation for all values.

[codecov-commenter]

Codecov Report

Merging #262 (bef7592) into main (53665a6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #262   +/-   ##
=======================================
  Coverage   16.20%   16.20%           
=======================================
  Files          89       89           
  Lines        7533     7533           
=======================================
  Hits         1221     1221           
  Misses       6295     6295           
  Partials       17       17           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 53665a6...bef7592. Read the comment docs.