Add input token constraints on output for integrity check

[jqueguiner]

Presentation of the new feature

In some cases especially on the NER use case it would be necessary to
Have for some fields an integrity flag to force
The generation to be picking the tokens among the input tokens only.

Where does it fit in Outlines?

Big use case for NER, avoid hallucinations and ensure integrity.

Are you willing to open a PR?

No

Thanks for contributing!