Force all secret properties to be string, required and non-nullable

[travjenkins]

Request

To make editing entities in the UI better we would like to narrow the scope of what we support for secret fields.

Non-Nullable

We do not want the field to be able to be null as that makes clearing the field during edit complex for the UI and UX. What we do not want to support:

"foo": {
  "title": "Example nullable string secret",
  "description": "",
  "type": [
    "string",
    "null"
  ],
  "secret": true
}

String

We'd like to keep the type to just string. This can be changed later if we deem it necessary. However, right now a secret number would not render correctly.

Required

I am still not 100% sure about this one... but probably makes most sense for anything that is a password - or password like - should always be there.

This is helpful for the UI because during edit we clear out all sops encrypted fields and if they are not required and user might not notice they did not fill that fields out again during edit.