add JsonnetNatevieFuncs

[fujiwara]

provides native functions for go-jsonnet.

funcs, _ := tfstate.JsonnetNativeFuncs(ctx, "", tfstateURL)
vm := jsonnet.MakeVM()
for _, fn := range funcs {
    vm.NativeFunction(fn)
}

local tfstate = std.native("tfstate");
{
  arn: tfstate("aws_acm_certificate.main.arn"), // string
  subject_alternative_names: tfstate("aws_acm_certificate.main.subject_alternative_names"), // array
  subject_alternative_names_0: tfstate("aws_acm_certificate.main.subject_alternative_names[0]"), // string
  tags: tfstate("aws_acm_certificate.main.tags"), // object
  tags_env: tfstate("aws_acm_certificate.main.tags").env, // string
}