feat: Reverse proxy support

[dadrus]

closes #4
closes #96

TODOs:

• implement the actual proxy handler
• refactor health and jwks endpoints to be served by a "management" port. Otherwise requests, which have the same path cannot be proxied and will be served by heimdall directly
• Docs: Update Quickstart-Guide to include Proxy usage, Docs: Update Config Description, ...
• Tests, Tests, Tests
• Implement "tracing middleware" for proxied requests
• Update the X-Forwarded-For header only if it is present. Otherwise use the Forwarded header, which superseded the X-Forwarded-For header
• Delete the X-Forwarded-Method, X-Forwarded-Proto, X-Forwarded-Host, X-Forwarded-Uri and X-Forwarded-Path from the request forwarded to the upstream service. These header are useless for it as these contain the URL parts, heimdall is using to connect.

[codecov]

Codecov Report

Merging #90 (b950d86) into main (1738623) will increase coverage by 0.46%.
The diff coverage is 81.13%.

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   85.02%   85.48%   +0.46%     
==========================================
  Files         141      146       +5     
  Lines        5221     5449     +228     
==========================================
+ Hits         4439     4658     +219     
- Misses        667      672       +5     
- Partials      115      119       +4     

Impacted Files	Coverage Δ
cmd/health.go	17.77% <0.00%> (ø)
internal/config/configuration.go	100.00% <ø> (ø)
internal/config/serve.go	0.00% <ø> (ø)
internal/fiber/middleware/xfmphu/handler.go	100.00% <ø> (ø)
internal/handler/management/jwks.go	100.00% <ø> (ø)
internal/handler/prometheus/module.go	0.00% <0.00%> (ø)
internal/heimdall/errors.go	100.00% <ø> (ø)
internal/x/compare.go	100.00% <ø> (ø)
internal/handler/proxy/module.go	42.37% <31.42%> (+42.37%)	⬆️
internal/handler/management/module.go	41.37% <41.37%> (ø)
... and 24 more

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