Created reply_in_json_interface and add json_interface_string utility

[Durbatuluk1701]

Not sure if you are wanting/accepting PRs, but I've found this reply_in_json_interface prompting utility to work well.

I noticed that reply_in_json was not yielding great results for my testing (notably on smaller models), as the model would occasionally ignore required interface fields or not respond with valid json.

This instead gives a more concrete, typescript style interface that the models seem to follow more consistently.

Example

class Address(BaseModel):
    street: str = Field(description="Street name and number")
    city: str
    zip_code: str

x = Address(....)
reply_in_json_interface(x)

yields:

Reply with the following JSON interface:
```
{
    street : string; // Street name and number
    city : string;
    zip_code : string;
}
```