libnm: add nm_dns_server_validate()

Add a new public symbol nm_dns_server_validate() that clients can use
to validate a single DNS server before adding it to the connection.

src/libnm-client-impl/libnm.ver

@@ -2076,3 +2076,8 @@ global:
 	nm_setting_sriov_get_preserve_on_down;
 	nm_sriov_preserve_on_down_get_type;
 } libnm_1_52_0;
+
+libnm_1_56_0 {
+global:
+	nm_dns_server_validate;
+} libnm_1_54_0;

src/libnm-core-impl/nm-setting-ip-config.c

@@ -4221,6 +4221,25 @@ nm_setting_ip_config_clear_dns(NMSettingIPConfig *setting)
     }
 }
 
+/**
+ * nm_dns_server_validate:
+ * @str: the string containing the DNS server
+ * @family: the IP address family (%AF_INET for IPv4, %AF_INET6 for IPv6,
+ *   %AF_UNSPEC to accept both IPv4 and IPv6)
+ * @error: (nullable): a pointer to %NULL #GError, or %NULL
+ *
+ * Validates a DNS name server string.
+ *
+ * Return: %TRUE if the name server is valid, %FALSE otherwise
+ *
+ * Since: 1.56
+ */
+gboolean
+nm_dns_server_validate(const char *str, int family, GError **error)
+{
+    return nm_dns_uri_parse(family, str, NULL, error);
+}
+
 GPtrArray *
 _nm_setting_ip_config_get_dns_array(NMSettingIPConfig *setting)
 {

src/libnm-core-public/nm-setting-ip-config.h

@@ -567,6 +567,9 @@ int nm_setting_ip_config_get_shared_dhcp_lease_time(NMSettingIPConfig *setting);
 NM_AVAILABLE_IN_1_54
 NMSettingIPConfigForwarding nm_setting_ip_config_get_forwarding(NMSettingIPConfig *setting);
 
+NM_AVAILABLE_IN_1_56
+gboolean nm_dns_server_validate(const char *str, int family, GError **error);
+
 G_END_DECLS
 
 #endif /* NM_SETTING_IP_CONFIG_H */