PL/Python: Refactor for event trigger support
Change is_trigger type from boolean to enum. That's a preparation for
adding event trigger support.
Author: Euler Taveira <euler@eulerto.com>
Co-authored-by: Dimitri Fontaine <dimitri@2ndQuadrant.fr>
Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/
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