Extend hcl2 support with more functions

[crazy-max]

Ref. tonistiigi/binfmt#14

More utility functions that could be useful in the HCL definition.

In a follow-up PR I would like to add value types accepted for a variable (currently limited to string type) like terraform does and maybe some basic validation:

variable "slugs" {
  type = list(string)
  default = [
    "crazymax/diun",
    "ghcr.io/crazy-max/diun"
  ]
  validation {
    condition = length(var.slugs) > 0
    error_message = "At least one slug is required."
  }
}

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com