feat(autonatv2): expose AllowPrivateAddrs via an Option

[gmelodie]

Summary

Expose AutoNATv2's allowPrivateAddrs setting through the top-level libp2p
package so callers can opt in without patching the library.

Changes

• p2p/protocol/autonatv2

  • Rename the private allowPrivateAddrs field/option to the exported
    AllowPrivateAddrs (commit e3a1d72d).

• config, options.go

  • Add Config.AutoNATv2AllowPrivateAddrs.
  • Add a new libp2p.AutoNATv2AllowPrivateAddrs() option.
  • When used alongside EnableAutoNATv2(), the fx provider passes
    autonatv2.AllowPrivateAddrs to autonatv2.New
    (commit b85ffade).

• libp2p_test.go

  • Add a smoke test verifying that:

    New(
        EnableAutoNATv2(),
        AutoNATv2AllowPrivateAddrs(),
    )

    successfully builds a host.

Motivation

nim-libp2p's AutoNATv2 interop tests currently maintain a patch against
go-libp2p to disable private-address filtering:

• disable-filtering-of-private-ip-addresses.patch
  (interop/autonatv2/go-peer/...)

The setting already exists internally; this change simply exposes it through the
public API so the patch can be removed (see nim-libp2p#1699).

Notes

• The option is documented as test-only and explicitly discouraged for
  production use.
• No behavioral change for existing callers; the default remains false.
• chore(autonatv2): remove go-libp2p patch from tests vacp2p/nim-libp2p#2558

[MarcoPolo]

Why does this field (and other similar fields across this diff) need to be public?