feat: Retrieval of an access token from the request body

[dadrus]

closes: #25

[codecov]

Codecov Report

Merging #115 (269df50) into main (362a55a) will increase coverage by 0.24%.
The diff coverage is 96.82%.

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
+ Coverage   85.55%   85.80%   +0.24%     
==========================================
  Files         146      147       +1     
  Lines        5449     5508      +59     
==========================================
+ Hits         4662     4726      +64     
+ Misses        668      664       -4     
+ Partials      119      118       -1     

Impacted Files	Coverage Δ
internal/pipeline/contenttype/decoder.go	100.00% <ø> (+25.00%)	⬆️
...tors/extractors/body_parameter_extract_strategy.go	95.74% <95.74%> (ø)
.../authenticators/extractors/mapstructure_decoder.go	68.42% <100.00%> (+1.14%)	⬆️
...ernal/pipeline/authenticators/jwt_authenticator.go	89.27% <100.00%> (+0.04%)	⬆️
...thenticators/oauth2_introspection_authenticator.go	91.81% <100.00%> (+0.03%)	⬆️
internal/pipeline/contenttype/json_decoder.go	100.00% <100.00%> (+50.00%)	⬆️
...ipeline/contenttype/www_form_urlencoded_decoder.go	100.00% <100.00%> (+100.00%)	⬆️

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