feat(generic): extra data and custom headers

[piksel]

Add support for @header=value and $data=value query parameters for the generic service.

This also enables support for header-based authentication, i.e. by using ?@authorization=Bearer+TOKEN.

Fixes #365
Fixes #366

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.21% 🎉

Comparison is base (f4d52db) 78.95% compared to head (52c041d) 79.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #377      +/-   ##
==========================================
+ Coverage   78.95%   79.17%   +0.21%     
==========================================
  Files         101      102       +1     
  Lines        4439     4485      +46     
==========================================
+ Hits         3505     3551      +46     
  Misses        756      756              
  Partials      178      178              

Files Changed	Coverage Δ
pkg/services/generic/custom_query.go	100.00% <100.00%> (ø)
pkg/services/generic/generic.go	86.36% <100.00%> (+0.99%)	⬆️
pkg/services/generic/generic_config.go	94.44% <100.00%> (+0.32%)	⬆️

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