Add source_pr_author template to backport action (#15394)

Allow substituting PR author in the title and description templates. This allows the PR title to mimic the message that GH would autogenerate in release notes, so backported PRs don't look out of place and don't have to be manually fixed.

.github/workflows/backport-base.yml

@@ -2,12 +2,12 @@ on:
   workflow_call:
     inputs:
       pr_title_template:
-        description: 'The template used for the PR title. Special placeholder tokens that will be replaced with a value: %target_branch%, %source_pr_title%, %source_pr_number%, %cc_users%.'
+        description: 'The template used for the PR title. Special placeholder tokens that will be replaced with a value: %target_branch%, %source_pr_title%, %source_pr_number%, %source_pr_author%, %cc_users%.'
         required: false
         type: string
         default: '[%target_branch%] %source_pr_title%'
       pr_description_template:
-        description: 'The template used for the PR description. Special placeholder tokens that will be replaced with a value: %target_branch%, %source_pr_title%, %source_pr_number%, %cc_users%.'
+        description: 'The template used for the PR description. Special placeholder tokens that will be replaced with a value: %target_branch%, %source_pr_title%, %source_pr_number%, %source_pr_author%, %cc_users%.'
         required: false
         type: string
         default: |
@@ -178,12 +178,14 @@ jobs:
               .replace(/%target_branch%/g, target_branch)
               .replace(/%source_pr_title%/g, context.payload.issue.title)
               .replace(/%source_pr_number%/g, context.payload.issue.number)
+              .replace(/%source_pr_author%/g, context.payload.issue.user.login)
               .replace(/%cc_users%/g, cc_users);
 
             const backport_pr_description = BACKPORT_PR_DESCRIPTION_TEMPLATE
               .replace(/%target_branch%/g, target_branch)
               .replace(/%source_pr_title%/g, context.payload.issue.title)
               .replace(/%source_pr_number%/g, context.payload.issue.number)
+              .replace(/%source_pr_author%/g, context.payload.issue.user.login)
               .replace(/%cc_users%/g, cc_users);
 
             // open the GitHub PR