python: Don't override message_schema for json schemas (#271)

I watched @amacneil today as he did this:

```python
chan = Channel("topic", message_encoding="msgpack", schema={ ... })
```

That doesn't work as expected, because we automatically override the
message_encoding to be json when you provide a schema that looks like a
dict jsonschema. Which is a silly thing to do, in retrospect. The schema
encoding does not imply a message encoding.

python/foxglove-sdk/python/foxglove/channel.py

@@ -27,7 +27,7 @@ def __init__(
         Create a new channel for logging messages on a topic.
 
         :param topic: The topic name.
-        :param message_encoding: The message encoding. Optional and ignored if
+        :param message_encoding: The message encoding. Optional if
             :py:param:`schema` is a dictionary, in which case the message
             encoding is presumed to be "json".
         :param schema: A definition of your schema. Pass a :py:class:`Schema`
@@ -118,11 +118,13 @@ def _normalize_schema(
     elif isinstance(schema, dict):
         if schema.get("type") != "object":
             raise ValueError("Only object schemas are supported")
-
-        return "json", Schema(
-            name=schema.get("title", "json_schema"),
-            encoding="jsonschema",
-            data=json.dumps(schema).encode("utf-8"),
+        return (
+            message_encoding or "json",
+            Schema(
+                name=schema.get("title", "json_schema"),
+                encoding="jsonschema",
+                data=json.dumps(schema).encode("utf-8"),
+            ),
         )
     else:
         raise ValueError(f"Invalid schema type: {type(schema)}")