[feature] Add support for BATMAN-adv and 802.11s mesh interfaces in O…

[OliverKruecken]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [x] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [x] I have updated the documentation.

Reference to Existing Issue

Closes #368

Description

This PR adds comprehensive support for BATMAN-adv (Better Approach To Mobile Ad-hoc Networking) mesh networking protocol and 802.11s wireless mesh interfaces to the OpenWrt backend.

Changes

New Interface Types

Implements three new interface types for the OpenWrt backend:

• batadv: Main BATMAN-adv virtual mesh interface

  • Configurable routing algorithm (BATMAN_IV, BATMAN_V)
  • Bridge loop avoidance settings
  • Gateway mode (off, client, server)
  • Hop penalty, MTU, and fragmentation controls

• batadv_hardif: Physical/hard interfaces for mesh participation

  • Device binding and master interface association

• mesh: Convenience alias for 802.11s wireless mesh interfaces (equivalent to batadv_hardif)

Schema Extensions

• Added JSON Schema definitions for all three interface types
• Added mesh-specific wireless properties (mesh_fwding, mesh_rssi_threshold)
• Added WPA3 Personal encryption support for mesh networks with CCMP cipher

Converter Implementation

• Implemented bidirectional conversion (NetJSON ↔ OpenWrt UCI) in the Interfaces converter
• Added forward conversion methods: _intermediate_batadv, _intermediate_batadv_hardif, _intermediate_mesh
• Added backward conversion methods: _netjson_batadv, _netjson_batadv_hardif, _netjson_mesh
• Updated WireguardPeers converter to properly filter blocks by type

Documentation

• Added comprehensive documentation to docs/source/backends/openwrt.rst (197 lines)
• Included configuration parameter reference tables
• Provided multiple working examples (basic and complete mesh setups)

Files Changed

• docs/source/backends/openwrt.rst (+197 lines)
• netjsonconfig/backends/openwrt/converters/interfaces.py (+58 lines)
• netjsonconfig/backends/openwrt/converters/wireguard_peers.py (+7 lines)
• netjsonconfig/backends/openwrt/schema.py (+116 lines)
• netjsonconfig/schema.py (+46 lines)

Total: 424 insertions, 1 deletion

Testing

• [x] All existing tests pass
• [x] New converter methods tested
• [x] Schema validation tested
• [x] Documentation examples verified

Backward Compatibility

• Maintains full backward compatibility with existing configurations
• No breaking changes to existing interface types
• Follows established netjsonconfig converter patterns