TypeListToStringSlice should only be allowed to be called on string lists

[ligfx]

Coming out of #172 – TypeListToStringSlice can currently be called on a types.List value with any internal element type. How can we make this more type-safe? Ideally this function is only ever called on types.List values that actually hold strings.

I saw another Terraform provider (was it Cloudflare's? hmm.) that had an interesting solution that defines generics that wrap types.List but also hold type information, and defines strongly-typed functions on those.

[ligfx]

Ah, it was Amazon's provider: https://github.com/hashicorp/terraform-provider-aws/blob/9002b22d2807d8385418a39e22b682f648b228cf/internal/framework/types/listof.go