[github] Add GitHub Plugin

[ricoberger]

Add a GitHub plugin, which can be used to view issues and pull requests
for a repository and organization. It is also possible to list
repositories and members of an organization and team. Users of kobs can
use the "userpullrequests" panel to add there created, assigned,
mentioned or review requested pull requests to their profile page.

The plugin requires a GitHub OAuth App to authorize users to access the
organizations data via the GitHub API.

• 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 #398 (52ad1ef) into main (a8a71d8) will increase coverage by 0.11%.
The diff coverage is 30.63%.

@@            Coverage Diff             @@
##             main     #398      +/-   ##
==========================================
+ Coverage   45.43%   45.55%   +0.11%     
==========================================
  Files         134      137       +3     
  Lines       10435    10634     +199     
==========================================
+ Hits         4741     4844     +103     
- Misses       5614     5707      +93     
- Partials       80       83       +3     

Impacted Files	Coverage Δ
cmd/kobs/hub/hub.go	0.00% <0.00%> (ø)
cmd/kobs/main.go	0.00% <0.00%> (ø)
cmd/kobs/satellite/satellite.go	0.00% <0.00%> (ø)
pkg/app/app.go	0.00% <0.00%> (ø)
pkg/hub/hub.go	0.00% <0.00%> (ø)
pkg/metrics/metrics.go	0.00% <0.00%> (ø)
pkg/satellite/satellite.go	0.00% <0.00%> (ø)
plugins/plugin-github/pkg/helpers/helpers.go	0.00% <0.00%> (ø)
plugins/plugin-github/cmd/github.go	28.28% <28.28%> (ø)
plugins/plugin-github/pkg/instance/instance.go	35.55% <35.55%> (ø)
... and 4 more

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