feat: Update A2A types from specification 🤖 (#179)

This PR updates `src/a2a/types.py` based on the latest
`specification/json/a2a.json` from
[google-a2a/A2A](https://github.com/google-a2a/A2A/commit/).

Author: a2a-bot

src/a2a/grpc/a2a_pb2.py

@@ -1,3 +1,5 @@
+import datetime
+
 from google.api import annotations_pb2 as _annotations_pb2
 from google.api import client_pb2 as _client_pb2
 from google.api import field_behavior_pb2 as _field_behavior_pb2
@@ -7,7 +9,8 @@ from google.protobuf.internal import containers as _containers
 from google.protobuf.internal import enum_type_wrapper as _enum_type_wrapper
 from google.protobuf import descriptor as _descriptor
 from google.protobuf import message as _message
-from typing import ClassVar as _ClassVar, Iterable as _Iterable, Mapping as _Mapping, Optional as _Optional, Union as _Union
+from collections.abc import Iterable as _Iterable, Mapping as _Mapping
+from typing import ClassVar as _ClassVar, Optional as _Optional, Union as _Union
 
 DESCRIPTOR: _descriptor.FileDescriptor
 
@@ -77,7 +80,7 @@ class TaskStatus(_message.Message):
     state: TaskState
     update: Message
     timestamp: _timestamp_pb2.Timestamp
-    def __init__(self, state: _Optional[_Union[TaskState, str]] = ..., update: _Optional[_Union[Message, _Mapping]] = ..., timestamp: _Optional[_Union[_timestamp_pb2.Timestamp, _Mapping]] = ...) -> None: ...
+    def __init__(self, state: _Optional[_Union[TaskState, str]] = ..., update: _Optional[_Union[Message, _Mapping]] = ..., timestamp: _Optional[_Union[datetime.datetime, _timestamp_pb2.Timestamp, _Mapping]] = ...) -> None: ...
 
 class Part(_message.Message):
     __slots__ = ("text", "file", "data")