Add support for registries config drop-in files

[jonesbusy]

Description

Add support for registries config drop-in files

Testing done

CI

Submitter checklist

• I have read and understood the CONTRIBUTING guide
• I have run mvn license:update-file-header, mvn spotless:apply, pre-commit run -a, mvn clean install before opening the PR

[codecov]

Codecov Report

❌ Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.71%. Comparing base (5733fdc) to head (414560f).

Files with missing lines	Patch %	Lines
src/main/java/land/oras/auth/RegistriesConf.java	82.14%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #753      +/-   ##
============================================
+ Coverage     87.33%   87.71%   +0.37%     
- Complexity     1095     1098       +3     
============================================
  Files            45       45              
  Lines          3380     3395      +15     
  Branches        457      458       +1     
============================================
+ Hits           2952     2978      +26     
+ Misses          253      242      -11     
  Partials        175      175              

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