feat: add is_finished method to RequestQueueClient interface

[Mantisus]

Description

• Adds an is_finished method to the RequestQueueClient interface. It's not a breaking change. The base class defaults is_finished to is_empty when it isn't overridden. Splits the queue predicates. is_empty means there are no requests available to fetch. is_finished means all requests are fully processed (nothing to fetch and nothing in progress).
• Implements is_finished in all built-in RequestQueueClient.
• Updates is_empty in all built-in RequestQueueClient to match the new meaning.
• Affects the AutoscaledPool. It now skips scheduling new tasks when no requests are available to fetch. For example, while the only request is being processed.

Issues

• Closes: Add an is_finished method to the StorageClient interface #1966
• Closes: Crawler can get softlocked when consuming the RequestQueue #1598

Testing

• Added a test for RequestQueue that checks the is_empty and is_finished values through the queue lifecycle. It covers all built-in clients.
• Added a regression test that checks the AutoscaledPool doesn't schedule new tasks while the only request is being processed.