Add notation sign CLI spec

[yizha1]

This PR is to create a new CLI spec for notation sign under a new directory named commandline. The notation-cli.md will be updated later to provide a list of notation commands and link each command to individual spec.

[codecov-commenter]

Codecov Report

Merging #341 (2c4e454) into main (8bec8a7) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #341      +/-   ##
==========================================
- Coverage   30.62%   30.54%   -0.08%     
==========================================
  Files          25       25              
  Lines        1610     1614       +4     
==========================================
  Hits          493      493              
- Misses       1104     1108       +4     
  Partials       13       13              

Impacted Files	Coverage Δ
cmd/notation/verify.go	18.62% <0.00%> (-0.38%)	⬇️
internal/cmd/signer.go	0.00% <0.00%> (ø)

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[priteshbandi]

LGTM.

cc. @gokarnm for review

[gokarnm]

LGTM! Except for expiry duration which I think we should switch to an expiry date time instead.

[shizhMSFT]

LGTM with suggestions