feat(python): expose prefilter to lancedb (#522)

We have experimental support for prefiltering (without ANN) in pylance.
This means that we can now apply a filter BEFORE vector search is
performed. This can be done via the `.where(filter_string,
prefilter=True)` kwargs of the query.

Limitations:
- When connecting to LanceDB cloud, `prefilter=True` will raise
NotImplemented
- When an ANN index is present, `prefilter=True` will raise
NotImplemented
- This option is not available for full text search query
- This option is not available for empty search query (just
filter/project)

Additional changes in this PR:
- Bump pylance version to v0.8.0 which supports the experimental
prefiltering.

---------

Co-authored-by: Chang She <chang@lancedb.com>

python/lancedb/table.py

@@ -844,9 +844,16 @@ class LanceTable(Table):
 
     def _execute_query(self, query: Query) -> pa.Table:
         ds = self.to_lance()
+        if query.prefilter:
+            for idx in ds.list_indices():
+                if query.vector_column in idx["fields"]:
+                    raise NotImplementedError(
+                        "Prefiltering for indexed vector column is coming soon."
+                    )
         return ds.to_table(
             columns=query.columns,
             filter=query.filter,
+            prefilter=query.prefilter,
             nearest={
                 "column": query.vector_column,
                 "q": query.vector,