netutils/netlib: support delete the DNS server address by index or address

[nuttxs]

Summary

netlib/netlib_delipv4dnsaddr.c: add DNS deletion functionality, support deleting DNS servers by address or by index:

netlib_del_ipv4dnsaddr()
netlib_del_ipv4dnsaddr_by_index()
netlib_del_ipv6dnsaddr()
netlib_del_ipv6dnsaddr_by_index()

Multi-DNS delete Call-Chain Implementation:
Application Layer: netlib_del_ipv4dnsaddr() / netlib_del_ipv6dnsaddr()
           ↓
Network Layer: dns_del_nameserver()
           ↓
Storage Layer: g_dns_servers[] array

Depend on PR#apache/nuttx#17284

Impact

New Feature/Change: Yes
User Impact: support delete the DNS server address by index or address
Build Impact:No new Kconfig options or build system changes.
Hardware Impact: No
Security: No
Compatibility: Backward-compatible; no breaking changes.

Testing

Test process example

// Add the first DNS server
netlib_set_ipv4dnsaddr(&dns1);

// Add the second DNS server
netlib_set_ipv4dnsaddr(&dns2);

// List all DNS servers
dns_foreach_nameserver(print_dns_callback, &count);

// Delete DNS server by index
netlib_del_ipv4dnsaddr_by_index(0);

// Delete DNS server by address
netlib_del_ipv4dnsaddr(&dns2);