Can't denote DuckDB table with custom schema using %sqlplot

[alexlowellmartin]

I've not been able to execute %sqlplot referencing a DuckDB table or view with a custom schema.

Ideally tables and views within a custom schemas could be referenced either automatically with schema_name.table_name syntax or with the addition of a --schema argument. (Or both)

Table with custom schema:

[edublancas]

hey @bbeat2782 this is similar to the PR we just merged, essentially --table schema.table should work, and --table table --schema schema as well

[bbeat2782]

Acceptance Criteria

1. Make sure --table schema.table and --schema schema --table table works for all plots in %sqlplot
2. Add test functions