Add new BlobOptimisticDataStore constructor

[gplwhite]

This allows customisation of the BlobContainerClient, such as specifying a managed identity credential.

var blobServiceUri = new Uri("https://snowmaker.blob.core.windows.net/datastore");
var blobContainer = new BlobContainerClient(blobServiceUri, new DefaultAzureCredential(), new BlobClientOptions() { Retry = { MaxRetries = 5 } });
var dataStore = new BlobOptimisticDataStore(blobContainer);

The existing connectionString based constructor remains as an overload.

[endintiers]

Works on my machine (tm) - I actually like the simplicity of just optionally passing the BlobContainer