Update exchanges GET API to account for new endpoint URL.

[pfeairheller]

This PR includes an update to exchanges GET API to account for new endpoint URL in KERIA.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (0b892d5) 90.47% compared to head (eb69e44) 90.39%.
Report is 1 commits behind head on development.

Files	Patch %	Lines
src/signify/app/credentialing.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           development      #99      +/-   ##
===============================================
- Coverage        90.47%   90.39%   -0.09%     
===============================================
  Files               16       16              
  Lines             1008     1010       +2     
===============================================
+ Hits               912      913       +1     
- Misses              96       97       +1     

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[m00sey]

lgtm