feat: supporting managed identity

[hallvictoria]

In order to support authentication through Managed Identity, the way the client is created needs to change. Previously it was done through the from_connection_string methods. This changes the extension to instead create a BlobServiceClient first, and then use the BlobServiceClient to create the respective SDK type.

When using Managed Identity, the only way to create a BlobClient is through a BlobServiceClient. There are two ways to create a BlobServiceClient:

1. Through the constructor: this is the only option when using Managed Identity
2. Through from_connection_string: this is the only option when not using Managed Identity

Since there are two options in how we can create the BSC, we have to track if MI is being used. This is done through a flag as part of the Client object. We determine if MI is being used based on the connection string setting passed in.

When using managed identity, the connection string variable name is formatted like so:

• Input: <CONNECTION_NAME_PREFIX>__serviceUri
• Trigger: <CONNECTION_NAME_PREFIX>__blobServiceUri

The variable received will be <CONNECTION_NAME_PREFIX>. Therefore, we need to append the suffix to obtain the storage URI and create the client.

There are four cases:

1. Not using managed identity: the environment variable exists as is
2. Using managed identity for blob input: __serviceUri must be appended
3. Using managed identity for blob trigger: __blobServiceUri must be appended
4. None of these cases exist, so the connection variable is invalid.