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/dynamodb.py

@@ -39,11 +39,11 @@ class DynamoBackend(Backend):
     async def close(self) -> None:
         self.client = await self.client.__aexit__(None, None, None)
 
-    async def get_with_ttl(self, key: str) -> Tuple[int, str]:
+    async def get_with_ttl(self, key: str) -> Tuple[int, Optional[bytes]]:
         response = await self.client.get_item(TableName=self.table_name, Key={"key": {"S": key}})
 
         if "Item" in response:
-            value = response["Item"].get("value", {}).get("S")
+            value = response["Item"].get("value", {}).get("B")
             ttl = response["Item"].get("ttl", {}).get("N")
 
             if not ttl:
@@ -56,12 +56,12 @@ class DynamoBackend(Backend):
 
         return 0, None
 
-    async def get(self, key: str) -> str:
+    async def get(self, key: str) -> Optional[bytes]:
         response = await self.client.get_item(TableName=self.table_name, Key={"key": {"S": key}})
         if "Item" in response:
-            return response["Item"].get("value", {}).get("S")
+            return response["Item"].get("value", {}).get("B")
 
-    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:
         ttl = (
             {
                 "ttl": {
@@ -83,7 +83,7 @@ class DynamoBackend(Backend):
             Item={
                 **{
                     "key": {"S": key},
-                    "value": {"S": value},
+                    "value": {"B": value},
                 },
                 **ttl,
             },