chore: updated warning printout logic for Sign with --allow-referrers-api flag

[Two-Hearts]

This quick PR updates the printout logic for sign, when user enables --allow-referrers-api.
The warning should be valid only when target artifact is stored in a registry.
If the user enables --oci-layout, i.e., target artifact is stored in an OCI layout, enabling --allow-referrers-api won't take any effect (this is because oci.Store does not support Referrers API yet). In this case, we shouldn't print out the warning.

The fix: During Sign, mark flags --oci-layout and --allow-referrers-api mutually exclusive.

Resolves #683.

[codecov-commenter]

Codecov Report

Merging #682 (0507d6e) into main (a695b60) will increase coverage by 0.01%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #682      +/-   ##
==========================================
+ Coverage   59.13%   59.15%   +0.01%     
==========================================
  Files          40       40              
  Lines        2183     2184       +1     
==========================================
+ Hits         1291     1292       +1     
  Misses        795      795              
  Partials       97       97              

Impacted Files	Coverage Δ
cmd/notation/sign.go	79.38% <100.00%> (+0.15%)	⬆️

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[priteshbandi]

LGTM

[priteshbandi]

Should we also add a check that --oci-layout and --allow-referrers-api cannot be used simultaneously?

[Two-Hearts]

Should we also add a check that --oci-layout and --allow-referrers-api cannot be used simultaneously?

@priteshbandi Then that would be an error for the user instead of dropping the --allow-referrers-api flag silently. (I'm okay with both of the options though.) /cc: @shizhMSFT @yizha1 @FeynmanZhou

[priteshbandi]

LGTM

[shizhMSFT]

LGTM