Add server configuration option to add default set of labels for workflows that has no labels specified

[lafriks]

Fixes #4324
Probably fixes #2634

Have not tested yet but wanted to open to get initial feedback

[woodpecker-bot]

Deployment of preview was torn down

[codecov]

Codecov Report

Attention: Patch coverage is 16.00000% with 21 lines in your changes missing coverage. Please review.

Project coverage is 27.37%. Comparing base (f4d7e9f) to head (3be2ff4).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
server/pipeline/items.go	0.00%	11 Missing ⚠️
cmd/server/setup.go	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4326      +/-   ##
==========================================
- Coverage   27.37%   27.37%   -0.01%     
==========================================
  Files         380      380              
  Lines       27781    27794      +13     
==========================================
+ Hits         7606     7608       +2     
- Misses      19490    19501      +11     
  Partials      685      685              

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[lafriks]

I'm already running this in our production (cherry-picked on v2.7.1) and currently all seems to work as expected so I think it's ready to review