Add support for OpenTofu

[appleoddity]

As of right now, OpenTofu is incompatible with SAM CLI. I spent quite a bit of time trying to shoehorn it in. It's possible to create a symlink from terraform -> tofu and get SAM CLI to generate plan data. However, the SAM CLI source code hardcodes the terraform registry so that it ignores resources created with modules from the opentofu registry. This includes null resources.

Because the serverless terraform-aws-modules creates a number of additional resources besides just the one that is defined in the IAC, it's not possible to control which module source it uses, specifically for the sam_metadata null resource.

This terraform/opentofu code generates a number of additional resources, including the sam_metadata resource:

module "lambda_function_auth" {
  source        = "registry.terraform.io/terraform-aws-modules/lambda/aws"
  version       = "~> 7.20.1"
  timeout       = 300
  source_path   = "../src/auth/"
  function_name = "authorizer"
  handler       = "app.handler"
  runtime       = "python3.11"
  create_sam_metadata = true
}

Even though I specify the terraform registry, the sub-resources this module creates from aws, external, local and null do not honor that. So, we get output like this in the plan JSON:

    {
      "address": "module.lambda_function_responder.null_resource.sam_metadata_aws_lambda_function[0]",
      "module_address": "module.lambda_function_responder",
      "mode": "managed",
      "type": "null_resource",
      "name": "sam_metadata_aws_lambda_function",
      "index": 0,
      "provider_name": "registry.opentofu.org/hashicorp/null",
      "change": {
        "actions": [
          "create"
        ],
        "before": null,
        "after": {
          "triggers": {
            "built_output_path": "builds/8854a8aeaa7f10e096468f95496525e5534711a51b64e026491ba7278c7546ad.zip",
            "original_source_code": "\"../src/responder/\"",
            "resource_name": "aws_lambda_function.this[0]",
            "resource_type": "ZIP_LAMBDA_FUNCTION",
            "source_code_property": "path"
          }
        },
        "after_unknown": {
          "id": true,
          "triggers": {}
        },
        "before_sensitive": false,
        "after_sensitive": {
          "triggers": {}
        }
      }
    }

Notice the provider_name.

At a minimum, the hardcoding of the terraform registry causes inflexibility and incompatibilities with using OpenTofu.

I propose you add an OpenTofu compatible hook.