feat: add option for Signed-Off-By line in upgrade action (#238)

Adds support for adding a `Signed-Off-By` line to commits created by the
upgrade action, as requested
[here](trunk-io/plugins#764).

Tested in this repo, you can see the commit
[here](cc7d39d).

upgrade/action.yaml

@@ -57,6 +57,11 @@ inputs:
     required: false
     default: false
 
+  signoff:
+    description: A boolean to add a Signed-off-by line to the commit message
+    required: false
+    default: false
+
 runs:
   using: composite
   steps:
@@ -122,3 +127,4 @@ runs:
         delete-branch: true
         reviewers: ${{ inputs.reviewers }}
         token: ${{ inputs.github-token }}
+        signoff: ${{ inputs.signoff }}