Add screenshare CBR rate control tests for V2 Video Encoder API.

Implement screenshare test cases in video_encoder_rate_control_unittest:
- ScreenshareSlideChangesFullHd: Evaluates 1080p slide presentations with
  mostly static content punctuated by intermittent slide changes every 2s,
  verifying bounded scene change bursts and compact static encoding.
- ScreenshareScrollingHd: Evaluates 720p scrolling over high-resolution
  content (1850x1110) with alternating scrolling and paused intervals,
  verifying CBR sliding window and total bitrate deviation.
- ScreenshareZeroHzHd: Evaluates Zero-Hz mode where a static frame is
  encoded at 1 Hz repeat rate while each frame's encoded CBR duration
  remains 1/target_fps, and subsequent resumption of 30 fps motion,
  verifying no oversized bursts on idle frames and quick convergence
  upon resuming motion.

Bug: webrtc:496266459
Change-Id: I0137fd256ab3daae29771fc508b4bd70a311acd0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/501920
Auto-Submit: Erik Språng <sprang@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48573}