[Task] API to use Managed Identity to authenticate against Cosmos DB

[Lybecker]

**Is your feature request related to a problem?

Currently the API use an API key STATE_STORE_KEY to authenticate against Cosmos DB. The API uses Managed Identity to authenticate against Azure Service Bus.

**Describe the solution you'd like

Using the same simplified access pattern to authenticate from the API to Azure resource.

Note: looks like Cosmos data-plane doesn't really support managed identity (in Python at least) but there might be a way to do this:
MicrosoftDocs/azure-docs#29762
https://docs.microsoft.com/en-us/azure/active-directory/managed-identities-azure-resources/tutorial-linux-vm-access-cosmos-db

[tamirkamara]

This is the best I came up with:

from azure.cosmos import CosmosClient
from azure.identity import DefaultAzureCredential
from azure.mgmt.cosmosdb import CosmosDBManagementClient

subscription_id = "..."
resource_group = "..."
account_name = "..."

credential = DefaultAzureCredential()
dbmgmt = CosmosDBManagementClient(credential, subscription_id, "https://management.azure.com")
keys = dbmgmt.database_accounts.list_keys(resource_group, account_name)

client = CosmosClient(url="...", credential=keys.primary_master_key)

[marrobi]

This is now possible in the SDK without using a master key, I will reopen and submit a PR