Add IPv6/dual-stack support: Operator-rendered and metrics server commands both hardcode 0.0.0.0 #6862

Is your feature request related to a problem? Please describe.

Yes, in two places, both hardcoding an IPv4-only bind:

1. start_metrics_server()
   binds via wsgiref.simple_server.make_server("", port, ...), which defaults
   socketserver.TCPServer.address_family to socket.AF_INET. On our IPv6-primary cluster,
   Istio's sidecar scrapes the pod's IPv6 address and gets connection refused — confirmed via
   Envoy access logs (503 delayed_connect_error:_Connection_refused on every scrape), while
   curl localhost:8000/metrics from inside the same container succeeds every time.

2. The Feast Operator's
   getContainerCommand()
   hardcodes -h 0.0.0.0 in the startup command of every service container it renders (online,
   ui, offline, lineage), with no CRD field to change it. On the same IPv6-only cluster, none
   of these servers bind an address anything can reach.

We work around (2.) today with a Kyverno ClusterPolicy that rewrites each container's command
at admission time — functional, but fragile: it doesn't self-heal already-running Deployments
(skipBackgroundRequests: true), and an earlier version of this exact policy replaced the entire
command with a hardcoded string instead of mapping over the operator's own argv, silently
dropping --workers, --metrics, and other flags for months before we caught it.

Describe the solution you'd like

feast.metrics.start_metrics_server() — bind dual-stack instead of IPv4-only:

import socket
from wsgiref.simple_server import WSGIServer, make_server

class DualStackWSGIServer(WSGIServer):
    address_family = socket.AF_INET6

    def server_bind(self):
        self.socket.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_V6ONLY, 0)
        super().server_bind()

httpd = make_server("::", port, app, server_class=DualStackWSGIServer)

Still works on IPv4-only hosts — Linux dual-stack sockets fall back cleanly with
net.ipv6.bindv6only=0, the default.

Feast Operator — add a BindAddress/DualStack field to ServerConfigs (parallel to other
per-service settings) that getContainerCommand() uses instead of the hardcoded 0.0.0.0
literal, accounting for the uvicorn-based ui/lineage servers rejecting a bracketed host (::)
unlike the gunicorn/Flight servers ([::]).

Describe alternatives you've considered

For the Operator half: the Kyverno ClusterPolicy above. It works, but every environment that
needs IPv6 needs Kyverno installed and a policy kept in lockstep with the Operator's own
command-building logic, and a bug in the policy (we've had one) doesn't self-heal already-running
Deployments.

Additional context

Test plan

• start_metrics_server() binds successfully and serves /metrics over both 127.0.0.1 and
  ::1 on a dual-stack host.
• Same binary still starts on an IPv4-only host (no IPv6 stack).
• A FeatureStore CR setting the new bind-address field renders the configured address in
  each affected container's command.