testplans: Add BoardPriority message.

- Starting point to allow setting board priorities for test
planning in config. Will usually be based off availability
in the lab. Can be set manually at first, and eventually
automatically updated based on current availability.

BUG=b:176490280
TEST=CQ

Change-Id: Idb44c06b4181df6ab4418437ec35f5cdb3351d8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2613344
Reviewed-by: C Shapiro <shapiroc@chromium.org>
Commit-Queue: Andrew Lamb <andrewlamb@chromium.org>
2 files changed