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]`.