fix: disallow multiple auth headers

[paullatzelsperger]

What this PR changes/adds

This PR disallows the use of multiple x-api-key headers to avoid Header Duplication Exploitation.

Why it does that

I honestly don't know why issuing 100 billion requests is harder than 1 billion requests with 100 headers each, but a
security report has been filed.

Further notes

.

Linked Issue(s)

Closes https://github.com/eclipse-tractusx/tractusx-edc/security/advisories/GHSA-wqpq-r4w2-5q5x

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a62205b) 72.10% compared to head (afedeed) 72.06%.
Report is 2 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3607      +/-   ##
==========================================
- Coverage   72.10%   72.06%   -0.04%     
==========================================
  Files         897      898       +1     
  Lines       17919    17973      +54     
  Branches     1018     1022       +4     
==========================================
+ Hits        12920    12952      +32     
- Misses       4562     4580      +18     
- Partials      437      441       +4     

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