screencast_portal: Allow hooks to adapt behavior for remote desktop

Change adds callbacks to the class so that the remote desktop portal can
still make use of this class for selecting sources but can provide its
own implementation on what to do after the sources are selected.
Furthermore, few getters are exposed in the class interface so as to
allow the remote desktop portal class to leverage them when sending the
captured pipewire frames onto the capture stream's consumer. Setters are
added for session, pipewire stream node id and few interfaces are made
public since remote desktop portal relies on them (e.g.
`SelectSources`).

The reason behind the change is that remote desktop portal depends on
screen cast portal for selecting sources. Also the setup to select
devices to control remotely as well as source selection should be
handled as part of the same session (and session should be
instantiated only once).

Currently, starting the screencast portal calls into a callback chain
that not only selects the sources but also starts the session but with
this change a consumer, such as remote desktop portal, can hook into
this callback chain by overriding the callbacks and provide a custom
callback chain from there onwards, if need be.

Bug: chromium:1291247
Change-Id: I983aff062ec2ddf52fdef5545fc58fede416e6ed
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/249862
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Salman Malik <salmanmalik@google.com>
Cr-Commit-Position: refs/heads/main@{#36285}