Decoupled code from DefaultSigningKey

[KN4CK3R]

Decoupled code from DefaultSigningKey. Makes testing a little bit easier and is cleaner.

[codecov-commenter]

Codecov Report

Merging #16743 (bd54abd) into main (4debb74) will increase coverage by 0.03%.
The diff coverage is 76.00%.

@@            Coverage Diff             @@
##             main   #16743      +/-   ##
==========================================
+ Coverage   45.37%   45.40%   +0.03%     
==========================================
  Files         760      760              
  Lines       85482    85484       +2     
==========================================
+ Hits        38785    38816      +31     
+ Misses      40414    40387      -27     
+ Partials     6283     6281       -2     

Impacted Files	Coverage Δ
services/auth/oauth2.go	49.18% <0.00%> (ø)
routers/web/user/oauth.go	37.38% <81.25%> (+0.14%)	⬆️
services/auth/source/oauth2/token.go	72.72% <85.71%> (ø)
modules/proxy/proxy.go	15.15% <0.00%> (-3.60%)	⬇️
modules/process/manager.go	72.83% <0.00%> (-2.47%)	⬇️
models/repo_list.go	77.64% <0.00%> (-0.79%)	⬇️
modules/queue/workerpool.go	54.96% <0.00%> (+0.38%)	⬆️
services/pull/pull.go	42.19% <0.00%> (+0.40%)	⬆️
routers/api/v1/repo/pull.go	29.63% <0.00%> (+0.51%)	⬆️
modules/queue/unique_queue_disk_channel.go	48.63% <0.00%> (+1.36%)	⬆️
... and 6 more

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[lunny]

Please resolve the conflicts.

[zeripath]

make lgtm work