Batch DtlsTransport lookups in SdpOfferAnswerHandler

This reduces the number of blocking calls to the network thread during
SDP negotiation by fetching all DtlsTransports for affected transceivers
in a single blocking call instead of one per transceiver.

In some situations this means a significant reduction of hops to the
network thread. Example from the
RenegotiateManyVideoTransceiversAndWatchAudioDelay test:

Before: Blocking ApplyLocalDescription: total=34
After:  Blocking ApplyLocalDescription: total=19

Bug: webrtc:42222804
Change-Id: I09d205959e18ef9c4c28db937fdd58fe44274d1b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/439360
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46551}