feat(auth): implement TokenSmith-backed service authentication

[alexlovelltroy]

• Introduced auth package to manage authentication modes: disabled, optional, and required.
• Added Provider struct to handle token exchange and auto-refresh logic.
• Integrated TokenSmith authentication into CoreSMD and CoreDNS plugins.
• Updated configuration parsing to include auth_mode, tokensmith_url, and refresh_before settings.
• Enhanced tests for authentication logic and configuration validation.
• Updated documentation to reflect new authentication features and usage examples.

Checklist

• My code follows the style guidelines of this project
• I have added/updated comments where needed
• I have added tests that prove my fix is effective or my feature works
• I have run make test (or equivalent) locally and all tests pass
• DCO Sign-off: All commits are signed off (git commit -s) with my real name and email
• REUSE Compliance:
  • Each new/modified source file has SPDX copyright and license headers
  • Any non-commentable files include a <filename>.license sidecar
  • All referenced licenses are present in the LICENSES/ directory

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

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