Add atmos terraform generate backends command

[aknysh]

what

• Add atmos terraform generate backends command

why

• Generate terraform backend configs for all terraform components
• Supported formats HCL and JSON
• A GitHub Action that generates all .tfvar files and backend.tf files so that projects can be used with conventional terraform GitOps tools like atlantis, Terraform Cloud, et al.

related

• Closes Feature Request: Generate All Terraform Backend Configurations #198

test

# hcl is default, no need to specify it
atmos terraform generate backends --format=hcl

Writing backend config for the terraform component 'test/test-component' to file 'examples/complete/components/terraform/test/test-component/backend.tf'
Writing backend config for the terraform component 'infra/vpc' to file 'examples/complete/components/terraform/infra/vpc/backend.tf'
Writing backend config for the terraform component 'top-level-component1' to file 'examples/complete/components/terraform/top-level-component1/backend.tf'

atmos terraform generate backends --format=json

Writing backend config for the terraform component 'test/test-component' to file 'examples/complete/components/terraform/test/test-component/backend.tf.json'
Writing backend config for the terraform component 'infra/vpc' to file 'examples/complete/components/terraform/infra/vpc/backend.tf.json'
Writing backend config for the terraform component 'top-level-component1' to file 'examples/complete/components/terraform/top-level-component1/backend.tf.json'