Concurrency for layer pull and push

[jonesbusy]

Description

Delegate pull/push layer to a dedicated Executor service with given parallelism.

Would improve performance for artifact of multiple layer or when copying from one Registry to an other

Testing done

Automated tests.

Will run more interactive tests using SNAPSHOT on main branch

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 85.98131% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.34%. Comparing base (79f879c) to head (149f41e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main/java/land/oras/Registry.java	80.00%	5 Missing and 4 partials ⚠️
src/main/java/land/oras/OCI.java	87.23%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #629      +/-   ##
============================================
+ Coverage     86.73%   87.34%   +0.60%     
- Complexity      902      914      +12     
============================================
  Files            43       43              
  Lines          2752     2773      +21     
  Branches        351      347       -4     
============================================
+ Hits           2387     2422      +35     
+ Misses          217      202      -15     
- Partials        148      149       +1     

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