Support customization/opting out of deserialization into DataFrames

[davidagold]

What

Redesmyn automatically deserializes incoming records into Polars DataFrames. In doing so, we assume the request payloads have a particular structure: a JSON array of stringified model record JSON objects against which to conduct inference. We should:

1. Support customization of the expected input data structure (it would be neat if we could enable users to declare the expected structure using Python type annotations),
2. Opt out of automatic deserialization entirely and directly receive a kind of Request object, or perhaps just a (possibly typed) dict.