Support Managed Database Read Replica Promotion

[christhemorse]

Description

This PR adds support for promoting Managed Databse read-only replicas to their own standalone primary subscriptions to match the functionality of the Vultr API and govultr library.

Checklist:

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

[github-actions[bot]]

Unit Tests and Coverage

Show Output

``` ? github.com/vultr/vultr-cli/v2 [no test files] ```

Pusher: @christhemorse, Action: pull_request_target

[github-actions[bot]]

Unit Tests and Coverage

Show Output

``` ? github.com/vultr/vultr-cli/v2 [no test files] ```

Pusher: @christhemorse, Action: pull_request_target

[github-actions[bot]]

Unit Tests and Coverage

Show Output

``` ? github.com/vultr/vultr-cli/v2 [no test files] ```

Pusher: @christhemorse, Action: pull_request_target