Feature Request: Add Cassandra as a kind for CosmosDB

Hello all,
I was looking to use [Terraform](https://www.terraform.io/docs/providers/azurerm/r/cosmosdb_account.html) to manage a CosmosDB cluster that uses the Cassandra API. I filed a ticket with them to add support for it, but they informed me in the [issue I filed](https://github.com/terraform-providers/terraform-provider-azurerm/issues/732) that this repository, as an upstream provider, would need to add support before they could add it to their golang-based tool.

I hope this issue can become a tracking mechanism for getting the Cassandra API `kind` added to CosmosDB. Thank you!

https://github.com/terraform-providers/terraform-provider-azurerm/issues/732

Start with the CosmosDB Cassandra API request in this issue and review the linked Terraform provider issue #732 for the expected downstream support. Then locate the relevant CosmosDB REST specification entry; done means the Azure REST specification represents Cassandra as a supported CosmosDB kind.

Written by the indexing model from the issue text.