event trigger support for PL/Python

Hi,

There is an old thread [1] that proposed $SUBJECT. That patch was not committed and the thread died. I use the referred
patchas base for the attached version. The key differences between them are: documentation, tests, refactor (0001) and
afew cleanups.
 


[1] https://www.postgresql.org/message-id/m2ob7wihex.fsf%402ndQuadrant.fr

--
Euler Taveira
EDB   https://www.enterprisedb.com/