Make caches Generic

We should consider making BaseCache Generic, so we can provide better type safety when relevant.

Code could then look something like this, with mypy checking:

cache: Cache[str] = Cache(...)
await cache.get("foo")  # -> str
await cache.set("foo", "bar")  # OK
await cache.set("foo", 45)  # Error: Expected str

Existing typing behaviour can be reproduced by annotating it with Cache[Any].

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