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

[nick-kang]

Exposing an architecture variable will allow you to specify arm64 AMIs.

In data "aws_ami"...

  filter {
    name   = "architecture"
    values = [var.architecture]
  }

In variables.tf...

variable "architecture" {
  type = string
  default = "x86_64"
}

Happy to submit a PR if you think this will be additive to the project.