.github/workflows: Add email validator workflow

This workflow validates that all commits in a pull request use email
addresses ending in @scylladb.com. For each commit with an author or
committer email that doesn't match this pattern, the workflow automatically
adds a comment to the pull request with a warning.

This serves two purposes:

1. Alert maintainers when external contributors submit code (which is
   acceptable, but good to be aware of)

2. Help ScyllaDB developers catch cases where they haven't configured
   their git email correctly

When a non-@scylladb.com email is detected, the workflow posts this
comment on the pull request:
```
    ⚠️ Non-@scylladb.com Email Addresses Detected

    Found commit(s) with author or committer emails that don't end with
    @scylladb.com.

    This indicates either:

    - An external contributor (acceptable, but maintainer should be aware)
    - A developer who hasn't configured their git email correctly

    For ScyllaDB developers:

    If you're a ScyllaDB employee, please configure your git email globally:

        git config --global user.email "your.name@scylladb.com"

    If only your most recent commit is invalid, you can amend it:

        git commit --amend --reset-author --no-edit
        git push --force

    If you have multiple invalid commits, you need to rewrite them all:

        git rebase -i <base-commit>
        # Mark each invalid commit as 'edit', then for each:
        git commit --amend --reset-author --no-edit
        git rebase --continue
        # Repeat for each invalid commit
        git push --force
```
Fixes: https://scylladb.atlassian.net/browse/RELENG-35

Closes scylladb/scylladb#27796

.github/workflows/call_validate_pr_author_email.yml

@@ -0,0 +1,13 @@
+name: validate_pr_author_email
+
+on:
+  pull_request_target:
+    types:
+      - opened
+      - synchronize
+      - reopened
+
+jobs:
+  validate_pr_author_email:
+    uses: scylladb/github-automation/.github/workflows/validate_pr_author_email.yml@main
+