Allow running postgres_query on a specific conn

[staticlibs]

This PR adds 2 new scalar functions:

• postgres_pin_connection(<attached_database_name>: VARCHAR) -> UBIGINT
• postgres_close_pinned_connection(<attached_database_name>: VARCHAR, <connection_id>: UBIGINT) -> BOOL

postgres_pin_connection acquires a connection from the connection pool of the specified attached database and "pins" that connection returning its ID as a handle. This handle can be specified to the postgres_query() function to make it run on that specific connection. This allows to make multiple subsequent postgres_query() and postgres_execute() calls on the same connection, thus keeping consistent server session state.

Pinned connection remains "active" and takes a slot in the connection pool until it is closed by calling postgres_close_pinned_connection.

Usage example:

ATTACH '...' AS s (TYPE postgres);

SET VARIABLE conn = postgres_pin_connection('s');

CALL postgres_execute('s', 'CREATE TEMPORARY TABLE temp1(col1 INT)', connection=getvariable('conn'));

CALL postgres_execute('s', 'INSERT INTO temp1 values(42)', connection=getvariable('conn'));

FROM postgres_query('s', 'SELECT * FROM temp1', connection=getvariable('conn'));
----
00:42:00

SELECT postgres_close_pinned_connection('s', getvariable('conn'));

Testing: query_pin_connection.test added.