feat(sigstore): add completion plugin

Closes #10862

Signed-off-by: Marco Franssen <marco.franssen@gmail.com>
Co-authored-by: Carlo Sala <carlosalag@protonmail.com>

plugins/sigstore/sigstore.plugin.zsh

@@ -0,0 +1,22 @@
+function install_autocompletion {
+  if (( ! $+commands[$1] )); then
+    return
+  fi
+
+  # If the completion file doesn't exist yet, we need to autoload it and
+  # bind it to `$1` (cosign, sget, rekor-cli). Otherwise, compinit will
+  # have already done that
+  if [[ ! -f "$ZSH_CACHE_DIR/completions/_$1" ]]; then
+    autoload -Uz _$1
+    typeset -g -A _comps
+    _comps[$1]=_$1
+  fi
+
+  $1 completion zsh >| "$ZSH_CACHE_DIR/completions/_$1" &|
+}
+
+install_autocompletion cosign
+install_autocompletion sget
+install_autocompletion rekor-cli
+
+unfunction install_autocompletion