Provide backend config values to TF code

[alt-dima]

To simplify "Data Source Configuration" (data "terraform_remote_state" "tfstate" { }) will be nice to have backend config values as tfvars.

var.tofugu_backend_config will contain all the parameters from TofuGu config (backend Section)

For example, for AWS S3:

data "terraform_remote_state" "network" {
  backend = "s3"
  config = {
    bucket = var.tofugu_backend_config.bucket
    key    = "network/terraform.tfstate"
    region = var.tofugu_backend_config.region
  }
}

And for GCS:

data "terraform_remote_state" "free_instance" {
  backend = "gcs"
  config = {
    bucket  = var.tofugu_backend_config.bucket
    prefix  = "account_free-tier/free_instance.tfstate"
  }
}