- fix check if function was already checked or not in passive mode fo…

…r PostgreSQL 18

- set correctly context of hiddent cast for all cases (not just for a…

…ssignment), and for this warning set the query field when the statement is EXECSQL

- fix build for PostgreSQL 18

- remove support for Postgres 12 and 13

- raise a warning when the expression is not pure expression

this is probably last release for PostgreSQL 12 and 13

- metadata and code cleaning

- minor changes

- allow to use custom pg_config

- disallow late pldbgapi2 initialization, fix some possible crashes

- fix another crash related unexpected order of fmgr hook calls