Add Unified Plan SDP performance benchmarks

Add BM_SdpDeserializeSmallUnifiedPlan, BM_SdpSerializeSmallUnifiedPlan
(3 media sections: 1 audio, 1 video, 1 data channel),
BM_SdpDeserializeLargeUnifiedPlan, and BM_SdpSerializeLargeUnifiedPlan
(50 media sections: 25 audio and 25 video with simulcast, RIDs, codecs,
header extensions, and candidates) using Google Benchmark in
api:webrtc_sdp_benchmark (linked into //:benchmarks).

Baseline metrics (Release with dcheck_always_on=false, 5-run median):
- BM_SdpDeserializeSmallUnifiedPlan: 85.9 us
- BM_SdpSerializeSmallUnifiedPlan: 20.8 us
- BM_SdpDeserializeLargeUnifiedPlan: 2167 us
- BM_SdpSerializeLargeUnifiedPlan: 518 us

Bug: none
Change-Id: I04ff95763997d8f71d31932e7ae8aabc4537dcdc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/509720
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48973}