support `any` filter

[GlenDC]

Consider:

#[derive(Debug, VennDB)]
pub struct Employee {
    #[venndb(filter, any)]
   pub country: Option<String>,
}

We need to find a way to generate code such that one can say:

let db = EmployeeDB::from_iter(/* ... */);
let mut query = db.query();
query.country("US".into());

// get all employees which are in US or have the "Any" country defined...
// whatever that means
let employees = query.exec()...