fix(redis): make xadd fields CovariantMapping

Using an InvariantMapping is wrong, the type of each values of the dict
can be different.

This change uses a Protocol instead to match as much as possible the
code:

```
if not isinstance(fields, dict) or len(fields) == 0:
    raise DataError("XADD fields must be a non-empty dict")
for pair in fields.items():
    pieces.extend(pair)
```

Author: sileht

stubs/redis/redis/commands/core.pyi

@@ -863,7 +863,7 @@ class StreamCommands:
     def xadd(
         self,
         name: KeyT,
-        fields: Mapping[bytes | memoryview | str | float, bytes | memoryview | str | float],
+        fields: SupportItems[bytes | memoryview | str | float],
         id: str | int | bytes | memoryview = "*",
         maxlen=None,
         approximate: bool = True,
@@ -929,7 +929,7 @@ class AsyncStreamCommands:
     async def xadd(
         self,
         name: KeyT,
-        fields: Mapping[bytes | memoryview | str | float, bytes | memoryview | str | float],
+        fields: SupportItems[bytes | memoryview | str | float],
         id: str | int | bytes | memoryview = "*",
         maxlen=None,
         approximate: bool = True,