Add a network slice property on Candidate.

For a local candidate, the new property will reflect the network slice
property for the Network from which this candidate was derived.

For a remote candidate, use of a non-default network slice could be
inferred from the network cost. But it is not currently possible to
infer the exact type of network slice used by a remote candidate.

Bug: webrtc:494142581
Change-Id: Ic00e7ced9a562750da85391f790f62e097043f1c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/461562
Reviewed-by: Jonas Oreland <jonaso@webrtc.org>
Commit-Queue: Sameer Vijaykar <samvi@google.com>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47365}