New destination for action:sendemail for notification handler

[cornelinux]

Is your feature request related to a problem? Please describe.

A user may have several different email addresses mapped, that can be used to send notifications to using the event handler. These email addresses can not be reached by the destination "To Email" of the event handler.

Describe the solution you'd like

We need a "To" mechanism like "To: Attribute", since the other email address is mapped to a different user attribute. Then the additional user attribute needs to be specified.

Additional context

See https://community.privacyidea.org/t/multiple-email-addresses-eventhandlerframework/3395/9