[Internal] Decouple oauth2 and serving (#855)

## What changes are proposed in this pull request?
This PR removes serving.py's indirect dependency on oauth2.py by moving
DataPlaneInfo to serving.py. This change is also reflected in the
OpenAPI specification. It narrows down import scopes to prevent circular
dependencies.

## How is this tested?
Existing Unit tests.

Author: parthban-db

.codegen/_openapi_sha

@@ -1 +1 @@
-a6a317df8327c9b1e5cb59a03a42ffa2aabeef6d
+779817ed8d63031f5ea761fbd25ee84f38feec0d

databricks/sdk/__init__.py

@@ -3,7 +3,6 @@
 from typing import Callable, List
 
 from databricks.sdk.oauth import Token
-from databricks.sdk.service.oauth2 import DataPlaneInfo
 
 
 @dataclass
@@ -19,6 +18,7 @@ class DataPlaneDetails:
 
 class DataPlaneService:
     """Helper class to fetch and manage DataPlane details."""
+    from .service.serving import DataPlaneInfo
 
     def __init__(self):
         self._data_plane_info = {}