update provider validation to allow a list of structs or a single struct

[bcarlock-mycarrier]

This validation allows the use of a list of structs or a single struct for provider definition. This means we can now define multiple instances of the same provider using standard Cue syntax, with no risk E.G.:

provider: azurerm: [
  {
    subscription_id: "USER INPUT SUBSCRIPTION"
    features: {}
  },
  {
    alias: "othersub"
    subscription_id: "SUBSCRIPTION"
    features: {}
  }
] 

Previously in my testing defining multiple instances of the same provider would cause the two providers to coalesce into a single instance if the subscription ID was the same and no alias is provided on the other instance. In our use case subscription 1 is derived from user input. Subscription 2 is a statically defined provider used to read data from some pre-defined azure resources. Subscription 1 and Subscription 2 may, in theory be the same subscription but must remain separate providers due to alias usage. This change preserves the said configuration of multiple provider instances.

Without the ability to use a list of structs for providers in the event that "USER INPUT SUBSCRIPTION" is equal to "SUBSCRIPTION" you end up rendering the above code down to this:

    "provider": {
        "azurerm":
            {
                "alias": "othersub",
                "subscription_id": "SUBSCRIPTION",
                "features": {}
            }
    },

With my proposed changes it becomes:

    "provider": {
        "azurerm": [
            {
                "subscription_id": "SUBSCRIPTION",
                "features": {}
            },
            {
                "alias": "othersub",
                "subscription_id": "SUBSCRIPTION",
                "features": {}
            }
        ]
    },