Add parameter to control the pacer's burst outside of field trials.

BurstyPacer is currently controlled via field trials. In order for
Chrome to be able to have burst without relying on a field trial, this
parameter is added.

When all burst experiments have concluded we may be able to have a
hardcoded constant instead, but for now the parameter is added to
RTCConfiguration.

NOTRY=True

Bug: chromium:1354491
Change-Id: I386c1651dbbcbf309c15ea3d3380cf8f632b5429
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/283420
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38621}

api/peer_connection_interface.h

@@ -695,6 +695,9 @@ class RTC_EXPORT PeerConnectionInterface : public rtc::RefCountInterface {
 
     PortAllocatorConfig port_allocator_config;
 
+    // The burst interval of the pacer, see TaskQueuePacedSender constructor.
+    absl::optional<TimeDelta> pacer_burst_interval;
+
     //
     // Don't forget to update operator== if adding something.
     //