PostgreSQL: add common.h

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: add query.h

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: add query_priv.h

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: add query.c

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: include headers

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: add query method

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: add postgresql.c

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: add connection.c

Signed-off-by: swpd <swpdtz@gmail.com>