[bug] udhcpc support is incomplete

## Steps to reproduce the problem

PR #4190 introduced udhcpc client support. As it is, udhcpc may work on some platforms, but is missing support for unknown dhcp options. This is used by microsoft azure to update the wireserver IP address via [option 245](https://github.com/canonical/cloud-init/blob/b729a4c4aa32d2bdfdd84fb24b9be5fc9c1eb293/cloudinit/sources/DataSourceAzure.py#L504). I believe that this is still a requirement, so this should be implemented for udhcpc as well for correct behavior on Microsoft Azure.