feat: Add change-description field to upgrade-one-dep workflow (#32214)

This will allow people to optionally describe the dependency upgrade
without having to rewrite the auto-generated commit or edit the PR body.
(The PR body isn't even guaranteed to make it into the eventual merged
commit.)

.github/workflows/upgrade-one-python-dependency.yml

@@ -12,6 +12,11 @@ on:
          description: 'Name of package to upgrade'
          required: true
          type: string
+       change_desc:
+         description: |
+           Description of change, for commit message and PR. (What does the new version add or fix?)
+         default: ''
+         type: string
 
 defaults:
   run:
@@ -39,13 +44,29 @@ jobs:
           make upgrade-package package="$PACKAGE"
 
       - name: PR preflight
+        env:
+          CHANGE_DESC: "${{ inputs.change_desc }}"
         run: |
           if git diff --exit-code; then
               # Fail early (and avoid quiet failure of create-pull-request action)
               echo "Error: No changes, so not creating PR." | tee -a "$GITHUB_STEP_SUMMARY"
               exit 1
           else
-              : # do nothing; exit code 1 means there are changes (good!)
+              # There are changes to commit, so prep some info for the PR.
+
+              # This is honestly a lot to go through just to say "add two newlines
+              # on the end if the variable isn't empty" but I guess this is what we
+              # have to do to get a conditional delimiter.
+              if [[ -z "$CHANGE_DESC" ]]; then
+                  echo "body_prefix=" >> "$GITHUB_ENV"
+              else
+                  EOF=$(dd if=/dev/urandom bs=15 count=1 status=none | base64)
+                  echo "body_prefix<<$EOF" >> "$GITHUB_ENV"
+                  echo "$CHANGE_DESC" >> "$GITHUB_ENV"
+                  echo "" >> "$GITHUB_ENV"
+                  echo "" >> "$GITHUB_ENV"
+                  echo "$EOF" >> "$GITHUB_ENV"
+              fi
           fi
 
       - name: Make a PR
@@ -58,9 +79,10 @@ jobs:
           commit-message: |
             feat: Upgrade Python dependency ${{ inputs.package }}
 
-            Commit generated by workflow `${{ github.workflow_ref }}`
+            ${{ env.body_prefix }}Commit generated by workflow `${{ github.workflow_ref }}`
           title: "feat: Upgrade Python dependency ${{ inputs.package }}"
-          body: "PR generated by workflow `${{ github.workflow_ref }}` on behalf of @${{ github.triggering_actor }}."
+          body: |
+            ${{ env.body_prefix }}PR generated by workflow `${{ github.workflow_ref }}` on behalf of @${{ github.triggering_actor }}.
           assignees: "${{ github.triggering_actor }}"
 
       - name: Job summary