Add partial support for packed format

[Firehed]

Adds partial support for packed attestation - only the self-attested format. This is used by, at least, macOS Edge when not signed in.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (652d9f1) 82.29% compared to head (ffdfea0) 82.90%.

Files	Patch %	Lines
src/Attestations/Packed.php	80.76%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #58      +/-   ##
============================================
+ Coverage     82.29%   82.90%   +0.61%     
- Complexity      227      233       +6     
============================================
  Files            26       27       +1     
  Lines           672      702      +30     
============================================
+ Hits            553      582      +29     
- Misses          119      120       +1     

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