[Event Hubs] Implementation of partition manage for durable store for checkpoints using blob storage

[ShivangiReja]

This pr creates a separate plugin and implement partitionManager for durable store for checkpoints using Blob Storage.

TODO:

• Update README
• Add Sample
• Update docs

[chradek]

I wanted to make a couple high-level comments.

We aren't storing any checkpoint data in the body of a blob, we're storing it all as metadata. When you make a listBlobsFlat call, you can specify that you want the metadata returned on each blob as well:

await containerClient.listBlobsFlat({include: ['metadata']});

This lets us get the metadata for each blob without having to make individual calls to the Storage service for each blob.

Since we're storing the blobs with a known prefix, we can also include that to the listBlobsFlat call so we only get blobs that start with our prefix:

await containerClient.listBlobsFlat({
  include: ['metadata'],
  prefix: `${eventHubName}/${consumerGroupName}/`}
);

[ShivangiReja]

Few things need to be updated, will create another PR for this.