Convert the subnet form to DDF

[skateman]

As now we're in the Carbon world and this form has arrays of entities (DNS servers) displayed in a textarea, we can leverage the properly implemented FieldArray component:

Other than that, the form does the same as the cloud volumes or the provider forms, i.e. loads a schema from the API using an OPTIONS request.

Resolves #6861
Depends on #7635 ManageIQ/manageiq-api#1012

Blocked by missing API endpoints ManageIQ/manageiq-api#1013

[miq-bot]

Checked commit skateman@5caa641 with ruby 2.6.3, rubocop 0.82.0, haml-lint 0.35.0, and yamllint
6 files checked, 1 offense detected

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• 💣 💥 🔥 🚒 - Linter/Haml - Linter::Haml STDERR:

warning: parser/current is loading parser/ruby26, which recognizes
warning: 2.6.6-compliant syntax, but you are running 2.6.3.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.

[miq-bot]

This pull request is not mergeable. Please rebase and repush.

[kavyanekkalapu]

cc @DavidResende0

[kavyanekkalapu]

this is done in #7734, closing it.