feat(config): add support for environment variable placeholders in config

[wing328]

Summary by cubic

Add environment variable placeholder support in openapitools.json so configs can reference ${VAR} or ${env.VAR} for dynamic values. Updated docs and tests to cover usage.

• New Features
  • Replaces placeholders in strings, arrays, and nested objects.
  • Logs an error and leaves the token unchanged when the env var is missing.
  • README: documents env. and placeholders for repository queryUrl/downloadUrl.
  • Adds unit tests for replacement behavior and edge cases.

^{Written for commit 5325b17. Summary will update automatically on new commits.}

based on #1031