feature: optionally enhanced location validation

[tombuildsstuff]

This commit introduces a new optional feature, for the enhanced validation of Locations.

The Azure MetaData Service - that is: /metadata/endpoints?api-version=2018-01-01 returns information about the locations which are suppported on the Azure Instance that we're connected to. As such, this commit optionally caches this information with the intent of providing more granular validation - to avoid cases where an unsupported location is specified.

This allows Terraform to catch this error during terraform plan - rather than failing during terraform apply - which is a better user experience.

This functionality is disabled by default at this time - but can conditionally be enabled via the Feature Flag using the Environment Variable ARM_PROVIDER_ENHANCED_VALIDATION to true.

Example before/with this feature disabled:

 $ ARM_PROVIDER_ENHANCED_VALIDATION=false tf plan
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.

------------------------------------------------------------------------

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # azurerm_resource_group.test will be created
  + resource "azurerm_resource_group" "test" {
      + id       = (known after apply)
      + location = "chinanorth"
      + name     = "tom-dev99"

      + timeouts {
          + create = "60m"
        }
    }

Plan: 1 to add, 0 to change, 0 to destroy

Example with this feature enabled:

$ ARM_PROVIDER_ENHANCED_VALIDATION=true tf plan
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.

------------------------------------------------------------------------

Error: "chinanorth" was not found in the list of supported Azure Locations: "westus,westus2,eastus,centralus,centraluseuap,southcentralus,northcentralus,westcentralus,eastus2,eastus2euap,brazilsouth,brazilus,northeurope,westeurope,eastasia,southeastasia,japanwest,japaneast,koreacentral,koreasouth,indiasouth,indiawest,indiacentral,australiaeast,australiasoutheast,canadacentral,canadaeast,uknorth,uksouth2,uksouth,ukwest,francecentral,francesouth,australiacentral,australiacentral2,uaecentral,uaenorth,southafricanorth,southafricawest,switzerlandnorth,switzerlandwest,germanynorth,germanywestcentral,norwayeast,norwaywest"

  on main.tf line 5, in resource "azurerm_resource_group" "test":
   5: resource "azurerm_resource_group" "test" {

[tombuildsstuff]

note: this intentionally doesn't use Azure/go-autorest since we don't need it here (it's an unauthorized endpoint, and logging this isn't super valuable)

[katbyte]

LGTM 👍

[katbyte]

THou i wonder if we should add this property to the docs?

[tombuildsstuff]

THou i wonder if we should add this property to the docs?

I'll add a section to the changelog for now, if we turn this on by default then it's probably worth documenting more broadly

[ghost]

This has been released in version 2.11.0 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 = "~> 2.11.0"
}
# ... other configuration ...

[ghost]

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