bird3: add new bird3 package

bird3: add new bird3 package

This commit introduces the `bird3` package to OpenWrt's routing feed.
The package is primarily based on the existing `bird2` package, with
minor modifications to accommodate the updates and changes in BIRD
3.0.0. I have also added myself as the maintainer for this package.

BIRD 3.0.0 is a major release that brings multithreading support,
enabling better performance for protocols like BGP, BMP, RPKI, and
others. The internal rework has introduced significant changes to the
table and channel implementations, while protocol-level functionality
remains largely consistent with BIRD 2.16. Although there are minor
breaking changes in configuration and CLI, most existing setups should
be reusable.

Memory usage has increased in this version, with improvements planned
for subsequent releases. Migration documentation is available in
`doc/migration-bird3.md` from the official BIRD repository.

For more details, see the official announcement:
https://trubka.network.cz/pipermail/bird-users/2024-December/017973.html

Signed-off-by: Nick Hainke <vincent@systemli.org>