[FEATURE] Support non-nullable fields in schema expression

[kvnkho]

Is your feature request related to a problem? Please describe.

At the moment, the schema expression treats all fields as nullable. For example:

"name:[{firstname:str,middlename:str,lastname:str}],id:str,gender:str,salary:int"

allows firstname, middlename, and lastname to be null.

Describe the solution you'd like

"name:[{firstname:str!,middlename:str!,lastname:str}],id:str,gender:str,salary:int"

can indicate that firstname and middlename can't be null.

Additional context

Fugue uses the schema from triad so this change will likely need to be done upstream.