TUN-10798: Parse Quick Tunnel allowed mail rules

## Summary

- extract the existing recipient-rule parser and validation helpers from the OTP PoC
- normalize and deduplicate repeated or comma-separated exact email and `*@domain` rules
- reject malformed addresses, wildcard domains, and empty rule sets
- keep the implementation as dead code with no CLI flag, provisioning, auth handler, or transport wiring

## Testing

- `make test`
- `make lint`

## Jira

- [TUN-10798](https://jira.cfdata.org/browse/TUN-10798)