Improve atmos validate stacks and atmos describe affected commands

[aknysh]

what

• Improve atmos validate stacks and atmos describe affected commands
• Update docs
  • atmos validate stacks
  • alias: Multiple Provider Configuration in Atmos Manifests

why

• atmos validate stacks now detects if the same component in the same stack are defined in more than one Atmos stack manifest files. If such a misconfiguration is detected, the following error is shown:

  the Atmos component 'vpc' in the stack 'plat-ue2-dev' is defined in more than one top-level 
  stack manifest files: orgs/acme/plat/dev/us-east-2, orgs/acme/plat/dev/us-east-2-extras.
  Atmos can't decide which stack manifest to use to get configuration for the component in the stack.
  This is a stack misconfiguration.

• atmos describe affected now has better error messages and also executes atmos validate stacks before detecting the affected components and stacks. This prevents the issue when the same component in the same stack is defined in more than one stack manifest file, and the command used one file from the current local branch and the other file from the target branch, resulting in drift in affected components and stacks. Since this is a stack misconfiguration and is not permitted, atmos describe affected will display the error and exit

• alias: Multiple Provider Configuration in Atmos Manifests

  Atmos allows you to define multiple configurations for the same provider using a list of provider blocks and the alias meta-argument.

  The generated providers_override.tf.json file will have a list of provider configurations, and Terraform will use and override the providers as long as the aliased providers are defined in the Terraform component.

  For example:

  components: 
    terraform:
      vpc:
        providers:
          aws:
            - region: us-west-2
              assume_role:
                role_arn: "role-1"
            - region: us-west-2
              alias: "account-2"
              assume_role:
                role_arn: "role-2"