Enable stopping when call (/token) budget hit

[leondz]

Summary

It should be possible to stop a run after a certain budget has been hit, defined in either input tokens, output tokens, or calls. We could do this by tracking prompts posed, their lengths, and output returned, and checking within a generator or probe whether or not budget is reached. Exceeding budget could be raised to the harness causing the run to end.

Example

Motivation

We should help users keep control over how much work garak poses to their targets