Enable CallbackList constructor subscription and adopt in NetworkManager

Update the CallbackList utility to allow subscribers to be added
directly during instantiation. This adds new constructors that accept a
callback, optionally with a removal tag, reducing the boilerplate
required to set up subscriptions immediately after object creation and
allows for removing multi-step initialization (that in some cases
requires thread hops).

As an initial accompanying change, the NetworkManager
hierarchy—including NetworkManagerBase and BasicNetworkManager—has been
updated to provide constructors that accept an initial network-change
callback. This allows clients to subscribe to network updates at the
moment of manager creation, simplifying initialization patterns and
ensuring no events are missed during setup. Existing tests have been
refactored to utilize these new constructor-based subscriptions.

Bug: webrtc:42222117
Change-Id: If1b001bf36e6a246c3ea241b1685d79e3c3acaec
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/467740
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47570}