Add getters to DatagramConnection for ice creds

Without this, users must wait for the first IceCandidate, where the
username fragment and password are included, which is a pain.

Bug: chromium:443019066
Change-Id: I9958b0e98d43986fd61f00c727ca1dab6ec54e71
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/427364
Auto-Submit: Tony Herre <herre@google.com>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46287}