modbus: add modbus_proxy_router for multi-backend gateways

[yoyixms]

routes each request to the backend chosen by a resolver callback and relays the response, replying with a gateway exception when the unit is unknown or unreachable

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: yoyixms.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[cla-bot]

We require contributors to sign our Contributor License Agreement. In order for us to review and merge your code, please fill https://forms.gle/5635zjphDo5JEJQSA to get added. Your document will be manually checked by the maintainer. Be patient...