[RM-5823] Fix type mismatch when constructing cty value

[jaspervdj-luminal]

The TF loader uses two representations of the document:

1. The native Go fashion, e.g. []interface{} for arrays,
   map[string]interface{} for objects, etc.
2. A more typed version, using the cty library.

When dealing with variables, we must sometimes convert from (1) to (2). cty
requires lists to be homogenous. This is usually not a problem since the types
in the schemas enforce this; however, in some cases we don't have these schemas,
and this list ends up looking heterogenous:

[
    {
      rule_no    = 100
      cidr_block = "0.0.0.0/0"
    },
    {
      rule_no         = 101
      ipv6_cidr_block = "::/0"
    },
]

We work around this by using a tuple instead of a list which allows for
different types.