RLS: Add support for UNION ALL

[joshua-spacetime]

I.e. bag or multiset union. You may define multiple RLS filters on the same table, the semantics of which are that of a bag union. For example:

/// A user only has access to their row
#[spacetimedb::client_visibility_filter]
const USER_FILTER: Filter = Filter::Sql("
    SELECT * FROM user WHERE identity = @sender
");

/// But the module owner has access to everything
#[spacetimedb::client_visibility_filter]
const USER_FILTER: Filter = Filter::Sql("
    SELECT user.*
    FROM user JOIN st_module
    WHERE st_module.owner_identity = @sender
");

[bfops]

We think this is required for RLS if e.g. you want your query to be x < 25 || x > 50

[joshua-spacetime]

Note, this does not require that we add UNION ALL to Spacetime SQL, but it will need to be supported by the logical query plan and execution engine.

[joshua-spacetime]

Combining with #2442