PL/Python: Add result metadata functions

Add result object functions .colnames, .coltypes, .coltypmods to
obtain information about the result column names and types, which was
previously not possible in the PL/Python SPI interface.

reviewed by Abhijit Menon-Sen

src/pl/plpython/sql/plpython_spi.sql

@@ -95,10 +95,13 @@ SELECT join_sequences(sequences) FROM sequences
 
 CREATE FUNCTION result_nrows_test() RETURNS int
 AS $$
-plan = plpy.prepare("SELECT 1 UNION SELECT 2")
+plan = plpy.prepare("SELECT 1 AS foo, '11'::text AS bar UNION SELECT 2, '22'")
 plpy.info(plan.status()) # not really documented or useful
 result = plpy.execute(plan)
 if result.status() > 0:
+   plpy.info(result.colnames())
+   plpy.info(result.coltypes())
+   plpy.info(result.coltypmods())
    return result.nrows()
 else:
    return None