[app] Use generics for jwt package

[ricoberger]

We are now using generics for the jwt package, so that we can reuse the
logic for cookies which are generated in plugins.

We use the jwt package now in the GitHub and Jira plugin. This has the
advantage that the cookies are more secure now. Before this change we
only used a base64 encoded representation of the auth credentials. Now
we generating a signed JWT token to store the auth info.

• 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 #420 (635a0e1) into main (9408dec) will decrease coverage by 0.25%.
The diff coverage is 36.66%.

@@            Coverage Diff             @@
##             main     #420      +/-   ##
==========================================
- Coverage   45.85%   45.60%   -0.26%     
==========================================
  Files         141      139       -2     
  Lines       11906    11888      -18     
==========================================
- Hits         5459     5421      -38     
- Misses       6306     6328      +22     
+ Partials      141      139       -2     

Impacted Files	Coverage Δ
plugins/plugin-jira/pkg/instance/instance.go	0.00% <0.00%> (ø)
plugins/plugin-github/pkg/instance/instance.go	35.71% <27.27%> (-23.02%)	⬇️
pkg/hub/auth/auth.go	72.15% <100.00%> (ø)
pkg/hub/auth/jwt/jwt.go	96.42% <100.00%> (ø)

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