cli: support escaping delimiters for "connection.seconaries"

It's obviously a change in behavior. Now accept backslash for escaping
the whitespace+comma separators when setting "connection.secondaries".

Before:

  $ nmcli --offline connection add type ethernet con-name x connection.secondaries 'a b'
  Error: failed to modify connection.secondaries: the value 'a' is not a valid UUID.
  $ nmcli --offline connection add type ethernet con-name x connection.secondaries 'a\ b'
  Error: failed to modify connection.secondaries: the value 'a\' is not a valid UUID.

After:

  $ nmcli --offline connection add type ethernet con-name x connection.secondaries 'a\ b'
  Error: failed to modify connection.secondaries: the value 'a b' is not a valid UUID.

https://bugzilla.redhat.com/show_bug.cgi?id=2177209

src/libnmc-setting/nm-meta-setting-desc.c

@@ -5606,7 +5606,7 @@ static const NMMetaPropertyInfo *const property_infos_CONNECTION[] = {
                 .remove_by_idx_fcn_u32 = MULTILIST_REMOVE_BY_IDX_FCN_U32 (NMSettingConnection, nm_setting_connection_remove_secondary),
                 .remove_by_value_fcn =  MULTILIST_REMOVE_BY_VALUE_FCN (NMSettingConnection, nm_setting_connection_remove_secondary_by_value),
                 .validate2_fcn =        _multilist_validate2_fcn_uuid,
-                .strsplit_plain =       TRUE,
+                .strsplit_with_spaces = TRUE,
             ),
         ),
     ),