chore: move interface to SPI

[paullatzelsperger]

What this PR changes/adds

moves the CredentialServiceUrlResolver to the SPI package, where it belongs.

Why it does that

Briefly state why the change was necessary.

Further notes

List other areas of code that have changed but are not necessarily linked to the main feature. This could be method
signature changes, package declarations, bugs that were encountered and were fixed inline, etc.

Linked Issue(s)

Closes # <-- insert Issue number if one exists

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.68%. Comparing base (7f20ba5) to head (37aa66c).
Report is 204 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4102      +/-   ##
==========================================
+ Coverage   71.74%   74.68%   +2.94%     
==========================================
  Files         919      979      +60     
  Lines       18457    20088    +1631     
  Branches     1037     1131      +94     
==========================================
+ Hits        13242    15003    +1761     
+ Misses       4756     4591     -165     
- Partials      459      494      +35     

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