[python-experimental] fn + method signature improvements

[spacether]

This PR makes improvements that can enable method Schema._validate caching in the future if one wants to implement it.
One would need to:

• have path_to_item use a generator for the last piece of the path_to_item so duplicate items in the array will have the same hash on their path_to_item
• have Schema._validate return a dict where the key is empty tuple as path_to_item, and then after that result is returned, append the current path fragment onto all keys in path_to_schemas
• perhaps pass in a _hash_key=type(arg) argument in the validate method because hash(True) == hash(1) in python

Updates in this PR:

• InstantiationMetadata changed to ValidationMetadata and only used in validation
• ValidationMetadata changed to inherit from frozendict, making it immutable
• InstantiationMetadata no longer passed into __new__ and _from_openapi_data, insetad _configuration is passed in
• *args becomes arg where possible
• __new__ and _from_openapi_data signatures kept the same (_configuration passed in) though _from_openapi_data could be rewritten to pass in a parameter named configuration because dicts will always be passed into it as arg. But for __new__, dicts may be passed in as keyword arguments.

PR checklist

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