v0.18.0-beta.1

Features:

• Implement ModelSchemaExtractDatabase for the google.spanner and google.firestore engines, deriving database bindings from the googleSpannerTable and googleFirestoreCollection Causa extensions.

Chore:

• Adapt to @causa/workspace's removal of the context argument from _call and _supports. The context is now read from this._context in all function implementations and deferred calls.
• Adapt to @causa/workspace-core's removal of BackfillEvent.key. The Pub/Sub publisher no longer forwards an ordering key.
• Replace js-yaml with yaml in the schema transformation script.