Expose an `architecture` variable to allow for `arm64` AMIs.

Exposing an `architecture` variable will allow you to specify `arm64` AMIs.

In `data "aws_ami"...`
```terraform
  filter {
    name   = "architecture"
    values = [var.architecture]
  }
```

In `variables.tf`...
```terraform
variable "architecture" {
  type = string
  default = "x86_64"
}
```

Happy to submit a PR if you think this will be additive to the project.