Implement variadic options for building tools and schemas, to allow
easier customization of tool input schemas.
Also, make fields required, unless they are marked as "omitempty".
Change-Id: I7a29258fae3ec5e7ea4906ed30ed6750979de962
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668955
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>