[Feature]: implement `user_data` attribute

[aleprovencio]

Description

This exists in other providers and makes possible to use cloud-init

New or Affected Terraform Resources

No response

Potential Terraform Configuration

data "template_file" "user_data" {
  template = file("cloud-init.yml")
}

resource "linode_instance" "my_instance" {
  label     = "my_instance"
  image     = "linode/ubuntu22.04"
  region    = "us-east"
  type      = "g6-nanode-1"
  root_pass = var.root_password
  user_data = data.template_file.user_data.rendered
}