Add support for joined filters in mbql

[chasleslr]

Metabase allows filtering on fields from another table joined through a foreign key. It compiles the MBQL for such filter as follows:

["=", ["field", 27, {source-field: 11}], 4]

This differs from a "regular" filter on a direct table field:

["=", ["field", 11, null], 5]

Here, {source-field: 11} represents the fieldID of the foreign key in the local table. For example, if using Orders as the base table with a foreign key userID to Users with a name field that we wanted to filter on:

• Users.name has a fieldID = 27 in the example above.
• Orders.userID has a fieldID = 11 in the example above.

We should allow Filter subclasses to filter on fields on foreign tables.