feat: support provider: none to skip cloud auth in tf-checks

[clouddrove-ci]

Summary

• Adds provider: none as a documented option for tf-checks.yml
• Allows repos to run terraform validate/fmt/init without cloud credentials
• No logic changes needed — all auth steps are already guarded by provider-specific conditionals, so none naturally skips all auth

Use case

Repos that only need syntax/format validation (no plan) can use:

uses: clouddrove/github-shared-workflows/.github/workflows/tf-checks.yml@master
with:
  working_directory: './examples/basic/'
  provider: none

Test plan

• Verify existing workflows with provider: aws/azurerm/gcp/digitalocean are unaffected
• Verify provider: none skips all auth steps and runs validate/fmt successfully

[dverma-cd]

LGTM