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/redis.py

@@ -1,4 +1,4 @@
-from typing import Optional, Tuple
+from typing import Optional, Tuple, Union
 
 from redis.asyncio.client import Redis
 from redis.asyncio.cluster import RedisCluster
@@ -7,18 +7,18 @@ from fastapi_cache.backends import Backend
 
 
 class RedisBackend(Backend):
-    def __init__(self, redis: Redis[str] | RedisCluster[str]):
+    def __init__(self, redis: Union[Redis[bytes], RedisCluster[bytes]]):
         self.redis = redis
         self.is_cluster: bool = isinstance(redis, RedisCluster)
 
-    async def get_with_ttl(self, key: str) -> Tuple[int, str]:
+    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[bytes]]:
         async with self.redis.pipeline(transaction=not self.is_cluster) as pipe:
             return await pipe.ttl(key).get(key).execute()
 
-    async def get(self, key: str) -> Optional[str]:
+    async def get(self, key: str) -> Optional[bytes]:
         return await self.redis.get(key)
 
-    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:
         return await self.redis.set(key, value, ex=expire)
 
     async def clear(self, namespace: Optional[str] = None, key: Optional[str] = None) -> int: