Support Managed Kafka

[christhemorse]

Description

This PR adds support for the Kafka engine type under the Managed Database product umbrella. In addition, there are a couple of small changes to data types such as with PlanReplicas in the Database struct (since it's replaced by plan_brokers for Kafka) and MinValue and MaxValue under AvailableOption being converted from int to float. Those will have to be accounted for when updating the vultr-cli and terraform libraries. Also, I reordered the DatabaseAdvancedOptions struct so the PG ones are together first, followed by MySQL and Kafka, so we're not trying to alphabetize everything we add across all the engine types. Otherwise, there are just a number of new properties and endpoints to establish all the launch functionality of Managed Kafka.

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[optik-aper]

Looks good