brmldproxy: add package

[T-X]

Adds the package brmldproxy from https://github.com/T-X/brmldproxy

[T-X]

Note: I'd also file a pull-request / move it to OpenWrt's package repository in the future, as brmldproxy should be generally useful for other bridge use-cases outside of Gluon.

[T-X]

Changelog v2:

• updating brmldproxy with a patch that fixes accidentally blocked, egress on a proxy port non-IPv6 packets, like IPv4 or ARP: T-X/brmldproxy@4d7fdb1

[T-X]

Changelog v3:

• adding an example UCI file

[T-X]

Changelog v4:

• update brmldproxy to the latest Git version:
  • fixes minor compile warnings with "-Wextra"
  • introduces a brmldpbX bridge interface on top of the brmldpX dummy interface, to be able to use a proxied MLD querier from the Linux bridge in Gluon, to ease overall MLD querier configuration