laurenz/oracle_fdw ORACLE_FDW_2_3

Upgrade note:

When upgrading from an older version of oracle_fdw, make sure to run

ALTER EXTENSION oracle_fdw UPDATE;

after installing the extension.

Support PostgreSQL v13. Support for 9.2 and 9.1 is dropped.
Add a function oracle_execute to execute arbitrary SQL statements on Oracle. The statements must not return a result (e.g. DDL statements).
Add an option dblink for Oracle database links.
The option can be used on foreign tables or with IMPORT FOREIGN SCHEMA.
Patch by @nboullis.
Add a column option strip_zeros that automatically strips ASCII 0 characters from strings.
Per request from @Sascha8a.
New IMPORT FOREIGN SCHEMA options max_long, sample_percent and prefetch. These set the corresponding options on imported tables.
Suggested by @ferfebles.
Add support for isolation levels other than SERIALIZABLE.
This is mostly because Oracle's implementation of SERIALIZABLE is so buggy.
Patch by @ThinkJ001.

Fix bad results with uncorrelated subqueries.
This affects queries have a foreign scan with a filter based on a subplan.
Reported by @srakazmus.
Fix Internal oracle_fdw error: encountered unknown node type 144.
This can happen in plans involving tables with identity columns.
Reported by @todoubaba.
Don't throw an error if DML statements modify no rows.
This might be caused by a trigger on the Oracle side.
Reported by @radist-nt.
Fix NCHAR and NVARCHAR2 handling.
With single-byte Oracle character sets, this bug led to replacement characters being used for characters not in the database character set.
Reported by @srakazmus, and @chrullrich helped with the fix.
Report a proper error for INSERT ... ON CONFLICT on partitioned tables.
Report and fix by @ibarwick.
Fix INSERT ... RETURNING if a table partition is a foreign table.
It used to return NULL values by mistake.
Report and analysis by @ibarwick.