New Resource: azurerm_firewall_nat_rule_collection

[Lucretius]

This resolves #2224 by adding a firewall_nat_rule_collection resource. This code is mostly copied from the firewall_network_rule_collection with some additions made for the additional properties required by the NAT rule.

As I have never used this resource, my research led me to the following:

1. In the Azure Portal I could not find a way to create a NAT rule collection using Snat, even though it is an option in the Go SDK.
2. UDP and TCP are the only allowed protocols for Dnat, not sure about Snat but my assumption is that it can include ICMP (probably to receive/responding to a ping which uses the ICMP protocol)
3. For Dnat - every property included translated_address and translated_port are required. I am not sure about Snat as I can find minimal information on it's Azure implementation.

If anyone has more familiarity with this resource who can sanity check my research, I would be happy for it.

[tombuildsstuff]

LGTM - thanks for this @Lucretius

[ghost]

This has been released in version 1.28.0 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 = "~> 1.28.0"
}
# ... other configuration ...

[ghost]

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