Make dns record create priority and ttl flags opt

[optik-aper]

Description

• Make the priority and ttl flags on a DNS record optional
• Set default value of priority to -1, mirroring the API behavior

Related Issues

Resolves #467

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/v3 [no test files] ```

Pusher: @optik-aper, Action: pull_request_target