Push notifications

@pydantic.with_config(A2A_MODEL_CONFIG)
class PushNotificationConfig(TypedDict):
    """Configuration for push notifications.

    When the server needs to notify the client of an update outside of a connected session.
    """

    id: Required[UUID]
    """The ID of the push notification configuration."""

    url: Required[str]
    """The URL of the push notification configuration."""

    token: NotRequired[str]
    """The token of the push notification configuration."""

    authentication: NotRequired[SecurityScheme]
    """The authentication of the push notification configuration."""

{
  "id": "8f0b6df8-1f1e-4b2d-9b0a-6c5e7e4d3a2b",
  "url": "https://client.example.com/webhook/notifications",
  "token": "secure-webhook-token",
  "authentication": {
    "type": "http",
    "scheme": "bearer",
    "bearerFormat": "JWT",
    "description": "Server-issued JWT for this webhook audience"
  }
}

{
  "id": "8f0b6df8-1f1e-4b2d-9b0a-6c5e7e4d3a2b",
  "url": "https://client.example.com/webhook/notifications",
  "authentication": {
    "type": "apiKey",
    "name": "X-Webhook-Key",
    "in": "header"
  }
}

@pydantic.with_config(A2A_MODEL_CONFIG)
class PushNotificationAuthenticationInfo(TypedDict):
    """Authentication information for push notifications."""

    schemes: list[str]
    """A list of supported authentication schemes (e.g., 'Basic', 'Bearer')."""

    credentials: NotRequired[str]
    """Optional credentials required by the push notification endpoint."""

@pydantic.with_config(A2A_MODEL_CONFIG)
class TaskPushNotificationConfig(TypedDict):
    """Configuration for task push notifications."""

    id: Required[UUID]
    """The ID of the task push notification configuration."""

    push_notification_config: Required[PushNotificationConfig]
    """The push notification configuration of the task push notification configuration."""

    long_running: NotRequired[bool]
    """Flag indicating task expects to run longer than typical request timeouts.

    When True, the push_notification_config should be persisted for notifications
    across server restarts. Defaults to False if not specified.
    """

{
  "id": "43667960-d455-4453-b0cf-1bae4955270d",
  "longRunning": true,
  "pushNotificationConfig": {
    "id": "8f0b6df8-1f1e-4b2d-9b0a-6c5e7e4d3a2b",
    "url": "https://client.example.com/webhook/a2a-notifications",
    "token": "secure-client-token-for-task-aaa",
    "authentication": {
      "type": "http",
      "scheme": "bearer",
      "bearerFormat": "JWT"
    }
  }
}

POST /webhook/a2a-notifications HTTP/1.1
Host: client.example.com
Authorization: Bearer <server_jwt_for_webhook_audience>
Content-Type: application/json
X-A2A-Notification-Token: secure-client-token-for-task-aaa

{
  "id": "43667960-d455-4453-b0cf-1bae4955270d",
  "contextId": "c295ea44-7543-4f78-b524-7a38915ad6e4",
  "status": {
    "state": "completed",
    "timestamp": "2024-03-15T18:30:00Z"
  },
  "kind": "task"
}