Feat: CAWG callback signing

[redaranj]

Changes in this pull request

Adds support for creating a Signer that combines a standard + an identity signer to allow CAWG x509 signing.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• All applicable changes have been documented
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.10%. Comparing base (6cfe007) to head (e074438).

Files with missing lines	Patch %	Lines
...ary/src/main/kotlin/org/contentauth/c2pa/Signer.kt	83.33%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #95      +/-   ##
============================================
+ Coverage     71.72%   72.10%   +0.37%     
- Complexity      256      258       +2     
============================================
  Files            58       58              
  Lines          2161     2183      +22     
  Branches        228      234       +6     
============================================
+ Hits           1550     1574      +24     
+ Misses          482      478       -4     
- Partials        129      131       +2     

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