Add support for passing a pre-created SQLAlchemy engine to Client

[kziovas]

What kind of change does this PR introduce?

A simple but extremely useful feature adding support for initializing the vecs client with a pre-build SQLAlchemy engine. This way we can utilize secure connection methods instead of just the connection string. See the discussion linked below for details.

What is the current behavior?

Please link any relevant issues here:
Supabase issue supabase/vecs#111 comment

What is the new behavior?

• Client.__init__ now accepts an optional SQLAlchemy engine.
• create_client can take either connection_string or engine.
• Enables secure connections using Google Cloud SQL Connector without patching.