[EventHub] Fix keyerror issue in BlobCheckpointStore

[yunhaoling]

Addressing issue: #13060
stack overflow issue: https://stackoverflow.com/questions/63354884/azure-eventhubs-python-checkpointing-with-blob-storage-keyerror-issue-in-ev

also added a test resource storage blob v2 with data lake enabled to verify the fix

---

--- echoing the context here ---

The root cause of KeyError is that the list_blobs functionality when called on a v2 storage blob with data lake enabled (hierarchical namespace) will not only get the per-partition checkpoint/ownership but also get the parent blob node which contains no metadata.

To illustrate this better, let's say we have the following blob structures:

- fullqualifiednamespace (directory)
  - eventhubname (directory)
    - $default (directory)
        - ownership (directory)
          - 0 (blob)
          - 1 (blob)
          ...

in v2 storage with data lake enabled (hierarchical namespace), when the code was using prefix
{<fully_qualified_namespace>/<eventhub_name>/<consumer_group>/ownership to search for blobs, the {<fully_qualified_namespace>/<eventhub_name>/<consumer_group>/ownership directory itself would also be returned containing no metadata leading to the KeyError when we're trying to extract information.

What we want is the per-partition blob, the the fix is easy: we add a / at the end of the prefix search string such that list_blobs won't return the parent node.
{<fully_qualified_namespace>/<eventhub_name>/<consumer_group>/ownership/

(Checkpoint would encounter the same problem)

[yunhaoling]

/azp run python - eventhub - tests

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