Import supported backends

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.

fastapi_cache/types.py

@@ -1,3 +1,4 @@
+import abc
 from typing import Any, Awaitable, Callable, Dict, Optional, Tuple, Union
 
 from starlette.requests import Request
@@ -20,3 +21,21 @@ class KeyBuilder(Protocol):
         kwargs: Dict[str, Any],
     ) -> Union[Awaitable[str], str]:
         ...
+
+
+class Backend(abc.ABC):
+    @abc.abstractmethod
+    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[bytes]]:
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    async def get(self, key: str) -> Optional[bytes]:
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    async def set(self, key: str, value: bytes, expire: Optional[int] = None) -> None:
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    async def clear(self, namespace: Optional[str] = None, key: Optional[str] = None) -> int:
+        raise NotImplementedError