Capture the stop token upon termination

**Is your feature request related to a problem? Please describe.**
I have a list of stop tokens `["FOO", "BAR"]` and I would like to know which one triggered the halting mechanism.

**Describe the solution you'd like**
I want to see the corresponding token in the response object, on top of `reason: stop`/

**Describe alternatives you've considered**
Until now I have to increment max_tokens incrementally while the stop token is not spotted in the response.

**Additional context**
Add any other context or screenshots about the feature request here.