Enable UDP listener & proxy support

[mhofstetter]

extensions: enable udp proxy

This commit enables the UDP proxy related envoy extensions.

* `UDPProxy` UDP listener filter
* `HTTPCapsule` UDP session filter
* `DynamicForwardProxy` UDP session filter

bpf_metadata: introduce udp bpf_metadata factory & instance

Currently, configuring a UDP listener (Envoy listener with socket
address protocol set to `udp`) with a UDPProxy in Envoy may
fail due to Envoy missing privileges to bind the listener
address.

The reason is that the Cilium Proxy uses a "starter" that drops all
privileged capabilities before forking the actual Envoy process
(See https://github.com/cilium/proxy/pull/315 for more details.) Setting up the necessary socket options
is part of the Cilium `BPFMetadata` listener filter.

The current `BPFMetadata` listener filter can't be used as UDP listener
filter (different factory). Therefore, this commit introduces a UDP
capable version of the `BPFMetadata` listener filter. It's only purpose
is to setup the UDP listener socket correctly (IP_TRANSPARENT, SO_REUSEADDR,
...). It's currently NOT adding any BPF metadata related information to the
socket. This is not necessary as the Cilium network filters aren't yet available
for UDP either.

[sayboras]

We might need to backport this to v1.31, as main branch is now with envoy v1.32.

[mhofstetter]

We might need to backport this to v1.31, as main branch is now with envoy v1.32.

Thanks for the reminder Tam! #1070