Use custom control for availability zone selection

[kuosandys]

The cluster creation form for creation based on cluster apps currently uses a string input field for availability zone/failure domain configuration. For example, for AWS:

For GCP:

For Azure, we use an enum type for the equivalent field in the cluster app schema, and we render a dropdown selector for the zones:

Misconfiguration of this field could prevent the cluster app from being successfully installed. We should align this across providers and refrain from using string input fields in order to reduce the likelihood of misconfiguration.

Related roadmap issue: #1181

[marians]

I'm leaving this open. It's created for happa, but the cluster configuration UI is going to be ported to Backstage in some form.

[weatherhog]

@marians is it still the goal to port cluster config UI to Backstage? Or can this be closed?

[marians]

We want to port the UI to Backstage and this is still relevant.