Azure Event Hubs Checkpoint Store Blob

The `azure-eventhub-checkpointstoreblob` library provides a checkpointer implementation for Azure Event Hubs, using Azure Blob Storage as the persistent store. It integrates as a plug-in package with `EventHubConsumerClient` from `azure-eventhub` to manage checkpoints and partition ownership information. This is the synchronous version of the library; an asynchronous counterpart `azure-eventhub-checkpointstoreblob-aio` is also available. As part of the Azure SDK for Python, it follows a regular release cadence.