[ext] add protocol version to all messages (#16633)

## Summary & Motivation

Add `dagster_ext_version` field to all ext messages.

## How I Tested These Changes

New tests.

python_modules/dagster-ext/dagster_ext/__init__.py

@@ -38,6 +38,9 @@
 # ##### PROTOCOL
 # ########################
 
+# This represents the version of the protocol, rather than the version of the package. It must be
+# manually updated whenever there are changes to the protocol.
+EXT_PROTOCOL_VERSION = "0.1"
 
 ExtExtras = Mapping[str, Any]
 ExtParams = Mapping[str, Any]
@@ -61,8 +64,12 @@ def _param_name_to_env_key(key: str) -> str:
 
 # ##### MESSAGE
 
+# Can't use a constant for TypedDict key so this value is repeated in `ExtMessage` defn.
+EXT_PROTOCOL_VERSION_FIELD = "__dagster_ext_version"
+
 
 class ExtMessage(TypedDict):
+    __dagster_ext_version: str
     method: str
     params: Optional[Mapping[str, Any]]
 
@@ -670,7 +677,9 @@ def __init__(
         self._materialized_assets: set[str] = set()
 
     def _write_message(self, method: str, params: Optional[Mapping[str, Any]] = None) -> None:
-        message = ExtMessage(method=method, params=params)
+        message = ExtMessage(
+            {EXT_PROTOCOL_VERSION_FIELD: EXT_PROTOCOL_VERSION, "method": method, "params": params}
+        )
         self._message_channel.write_message(message)
 
     # ########################

python_modules/dagster/dagster/_core/ext/utils.py

@@ -10,6 +10,7 @@
 from typing import Iterator, Optional
 
 from dagster_ext import (
+    EXT_PROTOCOL_VERSION_FIELD,
     ExtContextData,
     ExtDefaultContextLoader,
     ExtDefaultMessageWriter,
@@ -177,8 +178,7 @@ def extract_message_or_forward_to_stdout(handler: "ExtMessageHandler", log_line:
     # exceptions as control flow, you love to see it
     try:
         message = json.loads(log_line)
-        # need better message check
-        if message.keys() == {"method", "params"}:
+        if EXT_PROTOCOL_VERSION_FIELD in message.keys():
             handler.handle_message(message)
     except Exception:
         # move non-message logs in to stdout for compute log capture