Attach runtime before calling scan for foreign table provider

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

In the table provider for `lance` in the `scan` method I sometimes have to do a small amount of I/O to load dataset metadata if the metadata is not already in the cache. The `scan` method is async so I assume this is ok. However, I use `spawn_blocking` to trigger this I/O and it fails because the runtime is not active.

**Describe the solution you'd like**

The runtime should be attached before calling `scan`.

**Describe alternatives you've considered**

Currently I work around this by forcing the metadata to be cached before creating the table provider but this is inconvenient.

**Additional context**

Can provide additional details (stack traces, lance reproducers, etc.) on request.

Start at the table provider's async `scan` entry point and the `spawn_blocking` path described in the issue. Reproduce a Lance metadata cache miss, trace when the runtime is attached relative to `scan`, and confirm the cache-miss path completes without the runtime error.

Scritto dal modello di indicizzazione a partire dal testo della issue.