feat: Simpler endpoint configuration

[dadrus]

This PR closes #273

With this PR an endpoint can not only be configured in a structured way, like

endpoint:
  url: http://foo.bar

but also like

endpoint: http://foo.bar

which will reduce the amount of yaml code, if only the actual URL is required to be configured and the remaining parts of the endpoint configuration can remain mechanism default.

[codecov]

Codecov Report

Merging #376 (1b53d74) into main (785b956) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
+ Coverage   86.41%   86.44%   +0.02%     
==========================================
  Files         193      193              
  Lines        7727     7744      +17     
==========================================
+ Hits         6677     6694      +17     
  Misses        897      897              
  Partials      153      153              

Impacted Files	Coverage Δ
internal/endpoint/mapstructure_decoder.go	84.74% <100.00%> (+1.72%)	⬆️
.../rules/mechanisms/authenticators/config_decoder.go	83.33% <100.00%> (+0.98%)	⬆️
...nal/rules/mechanisms/authorizers/config_decoder.go	81.25% <100.00%> (+2.67%)	⬆️
...rules/mechanisms/contextualizers/config_decoder.go	81.25% <100.00%> (+1.25%)	⬆️
...rnal/rules/provider/httpendpoint/config_decoder.go	80.00% <100.00%> (+1.42%)	⬆️

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