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 ensures that any syntax issues are caught early (by type checkers
and tests). Backends that are missing dependencies are skipped. By
importing, this exposed an issue where the redis type annotations
raised an exception, which has been fixed by using forward annotations.
To help avoid import dependency hell, the Backend ABC has been moved to
`fastapi_cache.types`. In the process, it has been made an actual ABC.
This signals to automated tools what names usually can be imported from
the package, as otherwise you'd get warnings like `"default_key_builder"
is not exported from module "fastapi_cache"`.
