[python] Use pydantic for embedding function persistence (#467)

1. Support persistent embedding function so users can just search using
query string
2. Add fixed size list conversion for multiple vector columns
3. Add support for empty query (just apply select/where/limit).
4. Refactor and simplify some of the data prep code

---------

Co-authored-by: Chang She <chang@lancedb.com>
Co-authored-by: Weston Pace <weston.pace@gmail.com>

python/pyproject.toml

@@ -9,7 +9,8 @@ dependencies = [
     "aiohttp",
     "pydantic",
     "attr",
-    "semver>=3.0"
+    "semver>=3.0",
+    "cachetools"
 ]
 description = "lancedb"
 authors = [{ name = "LanceDB Devs", email = "dev@lancedb.com" }]