Issue #3607617: Decouple orchestra_webform_example from orchestra_mail

Notifications are channel-agnostic, so the example should not require a specific channel. Uninstalling orchestra_mail cascaded and took the example (its workflows + running instances) down with it.

• Drop the orchestra_mail dependency from orchestra_webform_example. Its MailHook already only acts on orchestra_mail messages, so it stays a no-op when that module is absent (an optional enhancement, not a requirement).
• Ship the example request_processed / request_rejected content for the Easy Email channel as config/optional (installs only when both easy_email and this module are enabled). The rejected template prints the validators comment via [orchestra:var:validation.comment].

Depends on #3607623 (nested var-token) for the rejection comment to render. Follow-up to #3607508.