Add tool for generating field trial registry header

The tool will generate a C++ header with all field trials in
REGISTERED_FIELD_TRIALS. This registry will later be used while looking
up field trials from native code to ensure they have been properly
registered in accordance with the policy.

Bug: webrtc:14154
Change-Id: I29bf880735121034585c541c46ef19f617d0afb9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/276268
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Emil Lundmark <lndmrk@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38426}