Initialize TaskQueuePacedSender with configuration at construction

Update TaskQueuePacedSender to support construction-time configuration
by introducing a new constructor that accepts PacerConfig. This allows
RtpTransportControllerSend to initialize the pacer directly in its
constructor initializer list and removes the need for a separate
SetConfig call inside EnsureStarted().

Bug: none
Change-Id: I6a438bf0527a749b9fa92937de33d4d704ec0889
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/479520
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47920}