Support kwargs predictors (#2345)
This change allows a predictor input to be defined exclusively as
`**kwargs`. This is an experimental change that allows a model
to be used as a proxy where any inputs will be passed through
directly without modification.
```py
import json
def predict(**kwargs) -> str:
return json.dumps(kwargs)
```
This change restricts the use of `**kwargs` to functions/methods
that accept only that as an argument to avoid the complexity of
having to support both dynamic and fixed inputs.
The OpenAPI schema for such a model will be:
```json
{
"properties": {},
"additionalProperties": true,
"type": "object",
"title": "Input"
}
```
