buffet: Add support for "required" and "isRequired" parameters

Added support for "required" attribute for object types and
"isRequired" for command parameters and made all properties/parameters
optional by default.

BUG=brillo:354
TEST=`FEATURES=test emerge-link buffet`

Change-Id: Ie3c7607e4ac0319f8ed459875a823fed39890da9
Reviewed-on: https://chromium-review.googlesource.com/283646
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
14 files changed
tree: 8f36f5b4979f322c15c7a0e4029bf8f745c10c3b
  1. buffet/
  2. libweave/