Expose how the providers are being created. That way, users can more
easily extend or override the list of providers that they want to use.
For example, you need a new AWS provider version:
```nix
terraform.withPlugins (p: [
(p.mkProvider rec {
owner = "hashicorp";
provider-source-address = "registry.terraform.io/hashicorp/aws";
repo = "terraform-provider-aws";
rev = "v${version}";
sha256 = "0fa61i172maanxmxz28mj7mkgrs9a5bs61mlvb0d5y97lv6pm2xg";
vendorSha256 ="1s22k4b2zq5n0pz6iqbqsf6f7chsbvkpdn432rvyshcryxlklfvl";
version = "3.56.0";
})
])
```
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