sqlite driver should support attaching a database

[neilotoole]

Is your feature request related to a problem? Please describe.

Other drivers (e.g. Postgres) have inherent support for SQL SCHEMA (and usually CATALOG). SQLite doesn't have CATALOG support, but by attaching a database, it effectively has SCHEMA support.

Issue #270 exposes the ability to switch schema per query. But we can't implement this functionality for SQLite until there's a means to attach databases.

Describe the solution you'd like
It should be possible to attach databases to a SQLite source, at a minimum, after the source has been added, and maybe when creating the soruce. An example implementation might be:

# Attach when adding the source (maybe?)
$ sq add --driver=sqlite3 ./main.db --attach ./sales.db --attach-as sales

# Attach after the fact
$ sq src attach @sqlite ./sales.db
$ sq src attach @sqlite ./inventory.db --name inven

$ sq src detach @sqlite sales
$ sq src detach @sqlite inven

Note that the sq src attach form is only an example of a possible implementation. Maybe sq config edit @sqlite or such could work too.

Describe alternatives you've considered
N/A

Additional context

• Do we need a means to list the attached databases?
• Would the attached databases show up in sq src (get source details), or maybe sq src -j?
• How to store details in the config file? Maybe something like:

handle: "@sakila/sl3"
driver: sqlite3
location: sqlite3:///Users/neilotoole/work/sq/sq/drivers/sqlite3/testdata/sakila.db
extra:
  sqlite3:
    attach:
      - name: sales
        path: ./sales.db
      - name: inven
        path: ./inventory.db

• Should the path of the attached db be stored as an absolute filepath, or as relative to the main db file (whose filepath is already stored as an absolute path)?