datachannel: Add a MaxSendQueueSize() accessor in the API

Previous limits was only in a comment and users had no way to query it
from the API.

Bug: webrtc:13289
Change-Id: I6187dd9f9482bc3e457909c5e703ef1553d8ef15
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/235378
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35224}

api/data_channel_interface.cc

@@ -40,4 +40,8 @@ bool DataChannelInterface::negotiated() const {
   return false;
 }
 
+uint64_t DataChannelInterface::MaxSendQueueSize() {
+  return 16 * 1024 * 1024;  // 16 MiB
+}
+
 }  // namespace webrtc