Add tools_webrtc/summarize_external_commits.py

This script identifies the top 20 external committers and provide a
monthly summary of external contributions to the main branch over the
last three years. It filters out corporate domains and service accounts
to focus on external contributors.

One contributor has been treated specially in order to emphasize
the diversity of inputs.

Bug: None
Change-Id: Id1a3ba7d2e0e45e218449f098b788b38a469082e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/462542
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47390}