New Resources: azurerm_(resource_group|subscription)_template_deployment

[tombuildsstuff]

This PR introduces two new resources:

• azurerm_resource_group_template_deployment
• azurerm_subscription_template_deployment

Which allow deploying an ARM Template Deployment at both the Resource Group and Subscription level. Whilst we currently have a Template Deployment resource in the form of the azurerm_template_deployment field - there's several limitations to this:

• we don't track external changes to the Template Body, we instead rely on the local template content
• we don't support import
• we don't support parameters or outputs of non-string values
• we don't clean up the resources deployed by this Template Deployment during the deletion of this resource

As such, whilst this Template Deployment resource is usable, it's not ideal - and ultimately this leads to us recommending users deploy an ARM Template into it's own Resource Group, to allow the Resource Group deletion to clear up items which have been deployed.

---

Instead this PR introduces two new resources azurerm_resource_group_template_deployment and azurerm_subscription_template_deployment - which can be used to deploy an ARM Template at either a Resource Group or a Subscription level.

These new resources support specifying Parameters (and retrieving Outputs) as JSON blobs - so we support all of the available types returned from Azure (Strings, Ints, Bools, Objects etc) via the jsondecode interpolation function.

In addition the Resource Group Template Deployment now attempts to delete the resources provisioned by this Template Deployment when it's being deleted (there's a feature-toggle for this in the provider block). It's worth noting this is best-effort for the moment and likely requires further tweaking - but seems to work with a nested deployment.

Unfortunately we're unable to do the same thing for Subscription Template Deployments due to a lack of support in the Azure API for a generic delete method at this level.

Whilst the existing azurerm_template_deployment resource won't be going away anytime soon - we can supersede it in this release, deprecate it in 3.0 and then remove it in the future once there's been sufficient time to switch-over.

This PR also updates the Resources package to 2020-06-01, so that we can gain this new functionality.

Fixes #3408
Fixes #5706
Fixes #6561

[jackofallops]

Thanks @tombuildsstuff - one question and one minor typo(?), but otherwise LGTM 👍

[tombuildsstuff]

Tests pass:

[ghost]

This has been released in version 2.30.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 2.30.0"
}
# ... other configuration ...

[ghost]

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