feat(generic): add builtin JSON tpl

[piksel]

This adds some additional functionality to the generic service to make it a bit more useful.

You can now supply the template as json to format the message and title as a simple JSON-object. Combined with the ability to rename the keys in the payload, it should add compatibility with at least a few know services.

ref #256

[codecov]

Codecov Report

Merging #260 (19b1f7b) into main (39c19cb) will increase coverage by 0.10%.
The diff coverage is 89.18%.

@@            Coverage Diff             @@
##             main     #260      +/-   ##
==========================================
+ Coverage   77.85%   77.96%   +0.10%     
==========================================
  Files          96       96              
  Lines        2877     2900      +23     
==========================================
+ Hits         2240     2261      +21     
- Misses        466      467       +1     
- Partials      171      172       +1     

Impacted Files	Coverage Δ
pkg/services/generic/generic_config.go	93.54% <ø> (ø)
pkg/services/generic/generic.go	86.66% <89.18%> (+2.88%)	⬆️

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