network: fix NFTSet population when [DHCPPrefixDelegation] Assign=no (#40049)

network: fix NFTSet population when [DHCPPrefixDelegation] Assign=no (#40049)

When [DHCPPrefixDelegation] Assign=no, networkd creates routes instead of
addresses. These routes need to populate nftables sets for firewall
rules to work correctly.

This commit adds dhcp_pd_route_modify_nft_set() to handle NFT set
updates for DHCP-PD routes, similar to how address_modify_nft_set()
handles them for addresses.

Fixes: #38383