ATTACH/DETACH syntax to open catalogs.

[universalmind303]

Description

Currently there is no way through SQL to attach to a remote instance for hybrid execution, or to an alternative catalog.

We can do this through the \open command in the cli (or the -l flag)

> glaredb -l db_1 # create a new catalog `db_1`
>> create table test as select 1;
>> \open db2 # create a new catalog `db_2`
>> create table test as select 2;
>> \open glaredb://... # connect to a cloud instance

but currently it's impossible to do that through sql, so i couldn't do something like this without introducing a scripting language.

copy_from_multiple_catalogs.sql

ATTACH db_1;
copy (select * from test) to 'db_1_out.csv'

ATTACH db_2;
copy (select * from test) to 'db_2_out.csv'

> glaredb -q copy_from_multiple_catalogs.sql