feat(raw-unpack): Add --start-from-digest flag

[kevinh-canva]

Context

Add a small flag to raw unpack command to support unpacking from a specific layer.

Let me know if we want users to provide just the digest without sha256: prefix instead

Demo

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.92%. Comparing base (a058839) to head (d43acf0).
⚠️ Report is 72 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/umoci/raw-unpack.go	12.50%	6 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #675      +/-   ##
==========================================
+ Coverage   72.89%   73.92%   +1.03%     
==========================================
  Files          71       71              
  Lines        6164     4641    -1523     
==========================================
- Hits         4493     3431    -1062     
+ Misses       1213      755     -458     
+ Partials      458      455       -3     

Files with missing lines	Coverage Δ
cmd/umoci/raw-unpack.go	62.90% <12.50%> (-4.60%)	⬇️

... and 58 files with indirect coverage changes

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