Partner ID: allowing users to opt-out of the default Terraform Partner ID

[tombuildsstuff]

This PR introduces an opt-out for the Default "Terraform Partner ID" introduced in #4663

Whilst the partner_id field can be used for various purposes - this Default Terraform Partner ID is only used by Microsoft to better understand Terraform's usage (and is only sent when a custom Partner ID isn't specified) - more information can be found in this comment.

This PR allows users to opt out of this Default Partner ID either through the Provider Block, like so:

provider "azurerm" {
  version = "=1.36.1"
  disable_terraform_partner_id = true
}

or by setting the Environment Variable ARM_DISABLE_TERRAFORM_PARTNER_ID to true.

Fixes #4747

[tracypholmes]

LGTM once all checks have passed!

[ghost]

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