The header name is configurable, and defaults to `X-FastAPI-Cache`,
the value is either `HIT` or `MISS`.
Note that the header is not set at all when the cache is disabled.
This is, for the majority of backends, the native format anyway, and so
we save encoding and decoding when using the PickleCodec or if (in future)
a orjson Coder was to be added.
For the JsonCodec, the only thing that changed is the location where the
JSON data is encoded to bytes and decoded back again to a string.
Instead of assuming that the Request and Response injected keyword
arguments can be named `request` and `response`, use namespaced
keyword names starting with a double underscore.
By default the parameter names now start with `__fastapi_cache_` and so
are a) clearly marked as internal, and b) highly unlikely to clash with
existing keyword arguments. The prefix is configurable in the unlikely
event that the names would clash in specific cases.
Use the return annotation to decode cached data to the correct type.
This follows the same logic FastAPI uses to JSON request bodies.
For the PickleCoder, this is a no-op as pickle already stores type
information in the serialised data.
The namespace argument is positional and will never be `None` so should
not be marked as Optional. It is always a string, and the default is
to pass in an empty string.
