fix: only sign statements

[mrjoelkamp]

summary

• SignStatements attempts to sign attestation manifests that were already signed

Signing attestations on image at docker://notary:server
Error: failed to sign attestation: failed to sign attestation layer failed to create signed layer: failed to get DSSE media type: unknown predicate type ""

• only sign layers that have unsigned statements

[mrjoelkamp]

this seems to have unexpectedly broke a bunch of test cases 🤯 🤔

[whalelines]

Do we need a test covering this?

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 69.98%. Comparing base (203577e) to head (bb0843c).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
sign.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
+ Coverage   69.95%   69.98%   +0.02%     
==========================================
  Files          44       44              
  Lines        2533     2535       +2     
==========================================
+ Hits         1772     1774       +2     
  Misses        469      469              
  Partials      292      292              

Flag	Coverage Δ
unittests	69.98% <66.66%> (+0.02%)	⬆️

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