Default to None for dagster_sling_translator in sling_assets (#21340)

## Summary & Motivation

Right now the `dagster_sling_translator` argument of `sling_assets` defaults to `DagsterSlingTranslator()`. Per https://docs.python-guide.org/writing/gotchas/ a single object instance will be shared across *all* invocations of that function. Even if technically the current object is immutable or has no state this is Python and people monkeypatch all the time. Additionally, if someone changes these objects in the future and aren't aware of this usage, it will introduce subtle bugs later. We should instead do the safe, obvious thing and create a new instance on every invocation.

## How I Tested These Changes

BK

python_modules/libraries/dagster-embedded-elt/dagster_embedded_elt/sling/asset_decorator.py

@@ -5,6 +5,7 @@
     AssetSpec,
     BackfillPolicy,
     PartitionsDefinition,
+    _check as check,
     multi_asset,
 )
 from dagster._utils.security import non_secure_md5_hash_str
@@ -29,7 +30,7 @@ def get_streams_from_replication(
 def sling_assets(
     *,
     replication_config: SlingReplicationParam,
-    dagster_sling_translator: DagsterSlingTranslator = DagsterSlingTranslator(),
+    dagster_sling_translator: Optional[DagsterSlingTranslator] = None,
     name: Optional[str] = None,
     partitions_def: Optional[PartitionsDefinition] = None,
     backfill_policy: Optional[BackfillPolicy] = None,
@@ -80,6 +81,13 @@ def my_assets(context, sling: SlingResource):
     streams = get_streams_from_replication(replication_config)
     code_version = non_secure_md5_hash_str(str(replication_config).encode())
 
+    dagster_sling_translator = (
+        check.opt_inst_param(
+            dagster_sling_translator, "dagster_sling_translator", DagsterSlingTranslator
+        )
+        or DagsterSlingTranslator()
+    )
+
     return multi_asset(
         name=name,
         compute_kind="sling",