feat(notifications): add json template

[piksel]

This adds the ToJSON template function to notifications, allows the template data to JSON serialized and adds a built in default JSON template (simply {{ . | ToJSON }}).

Fixes #1478 and allows for better compatibility with #1523.

[codecov]

Codecov Report

Base: 66.20% // Head: 66.76% // Increases project coverage by +0.55% 🎉

Coverage data is based on head (1fcfd86) compared to base (14b235a).
Patch coverage: 92.15% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1542      +/-   ##
==========================================
+ Coverage   66.20%   66.76%   +0.55%     
==========================================
  Files          24       25       +1     
  Lines        2314     2365      +51     
==========================================
+ Hits         1532     1579      +47     
- Misses        682      685       +3     
- Partials      100      101       +1     

Impacted Files	Coverage Δ
pkg/notifications/json.go	92.00% <92.00%> (ø)
pkg/notifications/shoutrrr.go	74.64% <100.00%> (+0.17%)	⬆️

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