Update JsonType containers to read-based collections

[nateprewitt]

The current definition for JsonType is constrained to dict for map-like objects but that introduces issues when input is explicitly typed.

The type checker is able to infer typing correctly with:

payload = {"test": "test"}
r = requests.post(url, json=payload)

but not

payload: dict[str, str] = {"test": "test"}
r = requests.post(url, json=payload)

This is due to the varied Value typing on JsonType and the mutability of dict. Changing it to Mapping which only exposes read interfacea allows us to check covariant value types. This works because the json input is never touched. It's taken from input and passed directly to json.dumps resulting in bytes. There's no access point in the pipeline to modify it later.

We're going to start with this expansion of the type to see if it works broadly. The goal is to avoid Any for JsonType if at all possible.