T5518: Add basic MLD support

Currently VyOS has `protocol igmp` option to enable IGMP querier and reports through FRR's pimd.

I would like to add support for IPv6 as well since FRR's IPv6 multicast functionality has significantly improved.

Enabling both MLD and IGMP on a VyOS router will allow us to turn on multicast snooping on layer-3 switches in dual-stack networks.

Example commands:

```
// Enable on interface eth0
set protocols pim6 interface eth0

// Explicitly join multicast group ff18::1234 on interface eth1
set protocols pim6 interface eth1 mld join ff18::1234

// Explicitly join source-specific multicast group ff38::5678 with source address 2001:db8::1 on interface eth1
set protocols pim6 interface eth1 mld join ff38::5678 source 2001:db8::1
```

1 files changed, 102 insertions, 0 deletions

diff --git a/src/conf_mode/protocols_pim6.py b/src/conf_mode/protocols_pim6.py
new file mode 100755
index 000000000..6a1235ba5
--- /dev/null
+++ b/src/conf_mode/protocols_pim6.py
@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2023 VyOS maintainers and contributors
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 or later as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from ipaddress import IPv6Address
+from sys import exit
+from typing import Optional
+
+from vyos import ConfigError, airbag, frr
+from vyos.config import Config, ConfigDict
+from vyos.configdict import node_changed
+from vyos.configverify import verify_interface_exists
+from vyos.template import render_to_string
+
+airbag.enable()
+
+
+def get_config(config=None):
+    if config:
+        conf = config
+    else:
+        conf = Config()
+    base = ['protocols', 'pim6']
+    pim6 = conf.get_config_dict(base, key_mangling=('-', '_'),
+                                 get_first_key=True, with_recursive_defaults=True)
+
+    # FRR has VRF support for different routing daemons. As interfaces belong
+    # to VRFs - or the global VRF, we need to check for changed interfaces so
+    # that they will be properly rendered for the FRR config. Also this eases
+    # removal of interfaces from the running configuration.
+    interfaces_removed = node_changed(conf, base + ['interface'])
+    if interfaces_removed:
+        pim6['interface_removed'] = list(interfaces_removed)
+
+    return pim6
+
+
+def verify(pim6):
+    if pim6 is None:
+        return
+
+    for interface, interface_config in pim6.get('interface', {}).items():
+        verify_interface_exists(interface)
+        if 'mld' in interface_config:
+            mld = interface_config['mld']
+            for group in mld.get('join', {}).keys():
+                # Validate multicast group address
+                if not IPv6Address(group).is_multicast:
+                    raise ConfigError(f"{group} is not a multicast group")
+
+
+def generate(pim6):
+    if pim6 is None:
+        return
+
+    pim6['new_frr_config'] = render_to_string('frr/pim6d.frr.j2', pim6)
+
+
+def apply(pim6):
+    if pim6 is None:
+        return
+
+    pim6_daemon = 'pim6d'
+
+    # Save original configuration prior to starting any commit actions
+    frr_cfg = frr.FRRConfig()
+
+    frr_cfg.load_configuration(pim6_daemon)
+
+    for key in ['interface', 'interface_removed']:
+        if key not in pim6:
+            continue
+        for interface in pim6[key]:
+            frr_cfg.modify_section(
+                f'^interface {interface}', stop_pattern='^exit', remove_stop_mark=True)
+
+    if 'new_frr_config' in pim6:
+        frr_cfg.add_before(frr.default_add_before, pim6['new_frr_config'])
+    frr_cfg.commit_configuration(pim6_daemon)
+
+
+if __name__ == '__main__':
+    try:
+        c = get_config()
+        verify(c)
+        generate(c)
+        apply(c)
+    except ConfigError as e:
+        print(e)
+        exit(1)