Set up informational-only Presubmit-BoringSSL-Verified label

This label will be applied by the bot when BoringSSL CQ dry run passes.
This label is informational only and does not block submit.

This will enable convenient Gerrit Flows actions such as sending a CL
for review automatically when the dry run passes.

The full label name is Presubmit-BoringSSL-Verified for explicitness,
because I might add more "Verified" labels soonish for the Copybara
integration.

Change-Id: I718e8c88a504d2d7ded30807e0f8c3d4345d5d80
Reviewed-on: https://boringssl-review.googlesource.com/c/All-Projects/+/91287
Reviewed-by: Adam Langley <agl@google.com>