[th/routing-rule-suppress-prefixlength]
Add support for "suppress-prefixlength".
WireGuard's wg-quick uses that for avoiding routing-loops, see https://git.zx2c4.com/WireGuard/tree/src/tools/wg-quick/linux.bash?id=fffe613427d70a7470d34b6dedcab9ffb8b6b667#n195
We should have that attribute supported, so that this configuration can be achieved by the user manually. Optimally, we still require a out-of-the-box solution that don't require the user to configure such a rule.