Add constrained_json_whitespace_pattern to ServerArgs

[zifeitong]

CC @qeternity @max99x

Motivation

It's hard to find a single whitespace pattern that works for all models(see dottxt-ai/outlines#691). I think it's better to leave the choice to the users.

Related PR: #1277

Modifications

constrained_json_whitespace_pattern is added to ServerArgs with the default value set to None (uses outlines default value).

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.