Add custom data to AzureBlobLeaseDistributedLock

[rLindorfer]

Problem

Currently, it is not possible to add additional meta-information to a distributed lock:

• identity who acquired the lock
• time stamp when lock got acquired
• etc.

Proposed solution

I would be useful if it was possible to add additional data to the created blob, e.g. a JSON uploaded as blob content.

1. add new constructor parameter Stream content to store custom data
2. instead of simply uploading an empty stream in the CreateIfNotExistsAsync method, use the given content