Feature should support translating string expression into JoinTransform

[lindong28]

Users should be able to run the following code

f_price = Feature(
    name="price",
    dtype=types.Float32,
    transform="item_price_events.price",
    keys=["item_id"],
)

purchase_events_with_features = DerivedFeatureView(
    name="purchase_events_with_features",
    source=purchase_events_source,
    features=[
        f_price,
        f_total_payment_last_two_minutes,
    ],
    keep_source_fields=True,
)