Add a benchmark for RTC_LOG

Measures RTC_LOG with one sink that receives the default log line, for
literals and numbers, mixed string types and SDPs of different sizes. It
is the baseline for reducing the heap allocations in RTC_LOG.

Bug: none
Change-Id: If06c9caf549f762dd0e24e0bbb08b4dc9ab1420c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/509961
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48974}