Fix #1378 create new attestation signature in replace mode if not existent

[Syquel]

Summary

Currently it is not possible to use cosign attest with the --replace option if either the attestation artifact or the attestation predicate does not exist.
Adds the ability to cosign attest with the --replace option to create new attestation artifacts or append new predicates to an existing attestation artifact.

Ticket Link

Fixes #1378

Release Note

Fixed the ability of `cosign attest` with the `--replace` option to create new attestations

[codecov-commenter]

Codecov Report

Merging #2014 (d4616e4) into main (76184c4) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #2014   +/-   ##
=======================================
  Coverage   26.16%   26.16%           
=======================================
  Files         127      127           
  Lines        7422     7422           
=======================================
  Hits         1942     1942           
  Misses       5225     5225           
  Partials      255      255           

Impacted Files	Coverage Δ
pkg/oci/mutate/mutate.go	81.28% <0.00%> (ø)

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[Syquel]

Is there something left for me to do to get this PR merged?
Would be great to have it in the next release.