feat: expose specifying scanner filters via datafusion (#3458)

We are trying to build a Datafusion table provider in LanceDB and it
receives expressions as DF logical exprs. We can go DF expr -> substrait
-> DF expr but it seems simpler to just expose the API to accept filters
as DF exprs directly.

rust/lance/src/dataset/scanner.rs

@@ -534,7 +534,7 @@ impl Scanner {
         Ok(self)
     }
 
-    pub(crate) fn filter_expr(&mut self, filter: Expr) -> &mut Self {
+    pub fn filter_expr(&mut self, filter: Expr) -> &mut Self {
         self.filter = Some(LanceFilter::Datafusion(filter));
         self
     }