Provide option to use the configured FROM email address for the event if it exists when sending email confirmation / receipts from the backoffice event registration form

Many sites configure a general FROM email address which they configure to be used for their event confirmation / receipt emails. However, when staff use backoffice 'Register Event Participant' form to create or edit / update an event registration - we always use the logged in user's email address as the FROM address. This is probably not the best choice for most use cases since it's a 'personal' email address.

(Also, if staff uses the batch 'Update Pending Contributions' workflow to record completed Pay Later registrations - we use the event-configured FROM email - so the behavior is not consistent.)

Solution
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When using backoffice event registration form - if user checks 'Send Confirmation and Receipt' - display a dropdown <select> where they can chose the FROM email address. Dropdown should include all active email addresses for the logged in user, any configured DOMAIN level FROM emails, AND the configured FROM email for the selected event (if any). Event-configured email should be the default if it exists. Otherwise the default configured DOMAIN FROM email address is the default for this field.