Improve Resource Provider registration handling

[kon-angelo]

How to categorize this issue?

/area control-plane
/kind enhancement
/priority 3
/platform azure

What would you like to be added:
Improve Azure's resource provider registration handling.

When interacting with Azure APIs to create resources, we are interacting with a resource provider and its types (source). Currently for a user of the gardener-extension-provider-azure it is necessary to have all the required providers registered to his subscription prior shoot creation.

This could be thought as a regression since in the past Terraform used to try and register all available providers but this caused an issue with newer versions of the terraform-provider-azurerm in subscriptions whose settings did not allow the registration of certain providers and was resolved with the addition of this change. AFAIK, for terraform this option currently is all-or-nothing, meaning that we can't force TF to only try and register the required providers and instead TF will try to register all the providers it supports which makes removing the skip_provider_registration flag from the TF script hard to accomplish.

This means that we need to:

• define explicitly in the documentation the required providers we interact.
• create a mechanism to handle the registration.

Examples of such mechanism could be:

• continue letting the user handle it (but mention it explicitly in the documentation).
• infrastructure validation check for the required providers prior to calling TF.
• attempt the registration ourselves.

[gardener-robot]

@kon-angelo Label area/todo does not exist.