schema: New functions `tfvarsencode`, `tfvarsdecode` and `exprencode` (v1.8)

[radeksimko]

See hashicorp/terraform#34718

Worth noting these are actually terraform provider namespaced functions, unlike the existing core functions.

[dbanck]

We support these functions when the built-in Terraform provider is declared in the required_providers block. This is the preferred way to use these functions, as stated in the docs: https://developer.hashicorp.com/terraform/language/v1.8.x/functions/terraform-encode_tfvars

I raised hashicorp/terraform#34891 to add descriptions for all the functions.

Today, we don't implicitly load a provider schema when you add a namespaced function to your configuration, so declaring the provider first, is the preferred way for functions.

I've checked that everything works as expected, so I'm closing the issue as complete.