refactor!: Push notification changes

[kthota-g]

BEGIN_COMMIT_OVERRIDE
refactor!: Removes push_notifier interface from the SDK and introduces push_notification_config_store and push_notification_sender for supporting push notifications.

feat: Adds support for more than one push_notification_config per task.

feat: Adds support for List and Delete push notification configurations.

---

Following is an example for the changes required with new version:

Previously:

from a2a.server.tasks import InMemoryPushNotifier
...
...
httpx_client = httpx.AsyncClient()
request_handler = DefaultRequestHandler(
    agent_executor=CurrencyAgentExecutor(),
    task_store=InMemoryTaskStore(),
    push_notifier=InMemoryPushNotifier(httpx_client),
)
...
...

Now:

from a2a.server.tasks import InMemoryPushNotificationConfigStore, BasePushNotificationSender
...
...
httpx_client = httpx.AsyncClient()
push_notification_config_store = InMemoryPushNotificationConfigStore()
push_notification_sender = BasePushNotificationSender(httpx_client,
                        config_store=push_notification_config_store)
        
request_handler = DefaultRequestHandler(
    agent_executor=CurrencyAgentExecutor(),
    task_store=InMemoryTaskStore(),
    push_config_store=push_notification_config_store,
    push_sender=push_notification_sender
)
...
...

Release-As: 0.2.11

END_COMMIT_OVERRIDE

[holtskinner]

@kthota-g Can you add this to the PR description with the version this should be for?

Release-As: 0.2.9

[kthota-g]

@kthota-g Can you add this to the PR description with the version this should be for?

Release-As: 0.2.9

sg. There are additional interface changes required to merge along with this PR. I will tag the release once the PR for other changes are in.

[kthota-g]

@swapydapy can you review this one more time? The changes to support multiple configs are complete and have updated the description with changes required for migration.