Add flag to use datagram transport (without implementation)
Add flag to use datagram transport (without implementation)

Integration with datagram transport will come in next CLs.

NOTE that since we now have implemented negotiation for media transport, we can replace configuration flags with field trials, but it will be done later for both media and datagram transports.

Bug: webrtc:9719
Change-Id: Icf062d030899d53d5646977ba195d1634050704b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/137820
Commit-Queue: Anton Sukhanov <20de4e819f6c8cd20c51bf8af5b559e4a7c96828@webrtc.org>
Reviewed-by: Steve Anton <93f73658da16553b94c040887a439a155812e54e@webrtc.org>
Reviewed-by: Bjorn Mellem <fbd9b9fcff27ba9e617a0b2d416d05a7dfd43a01@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#27996}

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