Expose api key validation (#491)

* introduce ApiKey module alongside with types

* wire ApiKey module through clients

* rename api_key modules to api_keys

* extend ApiKey response model by missing fields

* pluralize ApiKeyModule, ApiKey and AsyncApiKey

* remove AI slop, actually validate API key

* make api key validation path a constant

* adapt tests

* backport | None notation to Optional[T]

* return None on invalid API key

* improve naming

* black .

* fix type

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Co-authored-by: Nick Holden <nick.r.holden@gmail.com>