feat: Initial slash command support for psql

[scsmithr]

db=> \d
                    List of relations
    Schema     |         Name          | Type  |  Owner  
---------------+-----------------------+-------+---------
 glare_catalog | columns               | table | unknown
 glare_catalog | databases             | table | unknown
 glare_catalog | schemas               | table | unknown
 glare_catalog | session_query_metrics | table | unknown
 glare_catalog | tables                | table | unknown
 glare_catalog | views                 | table | unknown
(6 rows)

db=> \l
                         List of databases
  Name   |  Owner  | Encoding | Collate | Ctype | Access privileges 
---------+---------+----------+---------+-------+-------------------
 default | unknown |          |         |       | 
(1 row)

db=> create external database my_pg from postgres options (connection_string = 'postgres://postgres:postgres@localhost/postgres');
CREATE DATABASE
db=> \l
                         List of databases
  Name   |  Owner  | Encoding | Collate | Ctype | Access privileges 
---------+---------+----------+---------+-------+-------------------
 default | unknown |          |         |       | 
 my_pg   | unknown |          |         |       | 
(2 rows)

Much of this is stubbing things out. This also did add a slightly different pattern for creating scalar funcs, but I think it's easier to look at. I'll switch over our other functions in another pr.

[greyscaled]

So cool!