dhcpv6: T8849: Add time-zone support for Kea DHCPv6 (#5190)

* dhcpv6: T8849: Add time-zone support for Kea DHCPv6

Add DHCPv6 option support for time zone (RFC4833 options
41 and 42). This includes both the POSIX-style TZ string
(`new-posix-timezone`) and the IANA time zone name
(`new-tzdb-timezone`).

* dhcpv6: T8849: Refactor per code-review suggestion

* dhcpv6: T8849: Reformat for compliance

Author: indrajitr

interface-definitions/include/dhcp/option-v6.xml.i

@@ -102,6 +102,17 @@
       </constraint>
       </properties>
     </leafNode>
+    <leafNode name="time-zone">
+      <properties>
+        <help>Time zone to send to clients. Uses RFC4833 options 41 and 42</help>
+        <completionHelp>
+          <script>timedatectl list-timezones</script>
+        </completionHelp>
+        <constraint>
+          <validator name="timezone" argument="--validate"/>
+        </constraint>
+      </properties>
+    </leafNode>
     <node name="vendor-option">
       <properties>
         <help>Vendor Specific Options</help>

python/vyos/kea.py

@@ -89,6 +89,14 @@ def _find_list_of_dict_index(lst, key='ip', value=''):
     return idx
 
 
+def _read_posix_timezone(tz_name):
+    try:
+        with open(f'/usr/share/zoneinfo/{tz_name}', 'rb') as f:
+            return f.read().split(b'\n')[-2].decode('utf-8').replace(',', '\\,')
+    except (FileNotFoundError, IOError, IndexError) as e:
+        raise ConfigError(f'Failed to read timezone data for: {tz_name}') from e
+
+
 def kea_test_config(process: str, config_path: str) -> tuple[bool, str]:
     result, output = rc_cmd(f'{process} -t {config_path}')
 
@@ -143,9 +151,7 @@ def kea_parse_options(config):
         )
 
     if 'time_zone' in config:
-        with open('/usr/share/zoneinfo/' + config['time_zone'], 'rb') as f:
-            tz_string = f.read().split(b'\n')[-2].decode('utf-8').replace(',', '\\,')
-
+        tz_string = _read_posix_timezone(config['time_zone'])
         options.append({'name': 'pcode', 'data': tz_string})
         options.append({'name': 'tcode', 'data': config['time_zone']})
 
@@ -289,6 +295,11 @@ def kea6_parse_options(config):
         if hosts:
             options.append({'name': 'sip-server-dns', 'data': ', '.join(hosts)})
 
+    if 'time_zone' in config:
+        tz_string = _read_posix_timezone(config['time_zone'])
+        options.append({'name': 'new-posix-timezone', 'data': tz_string})
+        options.append({'name': 'new-tzdb-timezone', 'data': config['time_zone']})
+
     cisco_tftp = dict_search_args(config, 'vendor_option', 'cisco', 'tftp-server')
     if cisco_tftp:
         options.append(