luci-proto-batman-adv: protocol "Batman VLAN Interface"

[wififreedom]

A batman-adv VLAN interface has protocol 'batadv_vlan' and allows configuration of option 'ap_isolation' for a batman-adv VLAN device.

This adds support for configuration of such an interface to LuCI.

• ✅ This PR is not from my main or master branch 💩, but a separate branch ✅
• ✅ Each commit has a valid ✒️ Signed-off-by: <my@email.address> row (via git commit --signoff)
• ✅ Each commit and PR title has a valid 📝 <package name>: title first line subject for packages
• ✅ Tested on: x86_64, OpenWrt OpenWrt 25.12.1, r32768-b21cfa8f8c, firefox ✅ ; aarch64_cortex-a53, ARMv8 Processor rev 4, OpenWrt 25.12.0 r32713-f919e7899d, firefox ✅
• ✅ Mention: @onemarcfifty wrote the support for the other luci-proto-batman-adv scripts
• ✅ Description: LuCI -> Network -> Interfaces -> Add New Interface... -> Protocol: "Batman VLAN Interface". This requires that an interface with protocol "Batman Device" has already been created. For more information, see https://openwrt.org/docs/guide-user/network/wifi/mesh/batman?s[]=open#ap_isolation_for_batman-adv_vlans and https://openwrt.org/docs/guide-user/network/wifi/mesh/batman?s[]=open#batman-adv_options_for_vlans

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[wififreedom]

I've provided feedback for the formailty check issue: openwrt/actions-shared-workflows#91