Add user agent header

[jonesbusy]

Description

Add user agent header

Testing done

CI and ensure by checking debug mode the header is sent

Headers: ****, User-Agent=[ORAS-Java-SDK/0.2.2-SNAPSHOT]} 

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

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.50%. Comparing base (9602ce1) to head (d3b07b8).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main/java/land/oras/utils/Const.java	71.42%	2 Missing ⚠️

❌ Your patch status has failed because the patch coverage (75.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #222      +/-   ##
============================================
- Coverage     85.55%   85.50%   -0.06%     
  Complexity      465      465              
============================================
  Files            33       33              
  Lines          1530     1538       +8     
  Branches        175      175              
============================================
+ Hits           1309     1315       +6     
- Misses          146      148       +2     
  Partials         75       75              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.