Add option to only request a frame interval change via OnOutputFormatRequest.

OnOutputFormatRequest(const absl::optional<VideoFormat>& format)
changed to
OnOutputFormatRequest(
      const absl::optional<std::pair<int, int>>& target_aspect_ratio,
      const absl::optional<int>& max_pixel_count,
      const absl::optional<int>& max_fps)

Decouples:
- Resolution and fps requests.
- Resolution requests from aspect ratio requests.

Bug: webrtc:9597
Change-Id: I6f44c91283cf5474c6531e55773d2257e2341063
Reviewed-on: https://webrtc-review.googlesource.com/95423
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24623}