Introduce attribute (interpolated) value based validation

[radeksimko]

This is related/complimentary to #1442 Validating types is basically a first stage of validating expressions, and so that should probably get implemented first.

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Context

We do validate attributes in various contexts - for example missing required ones, or unexpected attributes.

We do not currently don't provide any validation of values though - for example we do not flag up the following:

terraform {
  required_version = "invalid-constraint"
}

terraform {
  required_providers {
    aws = {
      source = "invalid-source"
    }
  }
}

resource "aws_lb_listener" "name" {
  default_action {
    type = "invalid-type"
  }
  load_balancer_arn = "foo"
}

Proposal

• hcl-lang: Ensure that expressions get walked at all when validating
• hcl-lang: Introduce the concept of value-based validation into the schema
• terraform-schema: Add value-based validation to core schema where applicable

Stretch Goal

Find a way of exposing validation in a machine-readable way out of providers, such that this is available for LS.