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/inmemory.py

@@ -8,7 +8,7 @@ from fastapi_cache.backends import Backend
 
 @dataclass
 class Value:
-    data: str
+    data: bytes
     ttl_ts: int
 
 
@@ -29,21 +29,21 @@ class InMemoryBackend(Backend):
                 return v
         return None
 
-    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[str]]:
+    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[bytes]]:
         async with self._lock:
             v = self._get(key)
             if v:
                 return v.ttl_ts - self._now, v.data
             return 0, None
 
-    async def get(self, key: str) -> Optional[str]:
+    async def get(self, key: str) -> Optional[bytes]:
         async with self._lock:
             v = self._get(key)
             if v:
                 return v.data
             return None
 
-    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:
         async with self._lock:
             self._store[key] = Value(value, self._now + (expire or 0))