Make backends store bytes instead of strings

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.

fastapi_cache/backends/__init__.py

@@ -4,15 +4,15 @@ from typing import Optional, Tuple
 
 class Backend:
     @abc.abstractmethod
-    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[str]]:
+    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[bytes]]:
         raise NotImplementedError
 
     @abc.abstractmethod
-    async def get(self, key: str) -> Optional[str]:
+    async def get(self, key: str) -> Optional[bytes]:
         raise NotImplementedError
 
     @abc.abstractmethod
-    async def set(self, key: str, value: str, expire: Optional[int] = None) -> None:
+    async def set(self, key: str, value: bytes, expire: Optional[int] = None) -> None:
         raise NotImplementedError
 
     @abc.abstractmethod