pgsql: PL/Python: Add event trigger support

PL/Python: Add event trigger support

Allow event triggers to be written in PL/Python. It provides a TD
dictionary with some information about the event trigger.

Author: Euler Taveira <euler@eulerto.com>
Co-authored-by: Dimitri Fontaine <dimitri@2ndQuadrant.fr>
Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Discussion: /messages/by-id/03f03515-2068-4f5b-b357-8fb540883c38@app.fastmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/53eff471c69dc8b0c01f046d3fdcc460eb90d0e5

Modified Files
--------------
doc/src/sgml/plpython.sgml | 76 +++++++++++++++++++++++++++
src/pl/plpython/expected/plpython_trigger.out | 27 ++++++++++
src/pl/plpython/plpy_exec.c | 42 +++++++++++++++
src/pl/plpython/plpy_exec.h | 1 +
src/pl/plpython/plpy_main.c | 11 ++++
src/pl/plpython/plpy_procedure.h | 1 +
src/pl/plpython/sql/plpython_trigger.sql | 24 +++++++++
7 files changed, 182 insertions(+)