feat: support datasets in other (sub) DBs

[betodealmeida]

Allow syncing models that are in different (sub) databases. Closes #73.

For example, the SQL Alchemy for a given database might be bigquery://project_A, but the user might want to add a model that is defined in project_B. Since Superset doesn't allow creating a physical dataset in cases like this, we create a simple virtual dataset defined by:

SELECT * FROM project_B.schema.table;