Add new specs for release blocker tests

Below spec will represent release blocker tests for green release:
- 'release_blocker': if true any failure will block canary release.
- 'bug_component': should not be empty if a test suite is
  release_blocker. Thus gardener could file bug to notify the owners.

Proposal doc: go/chrome-green-release-tests

BUG=1455536
TEST=generate_buildbot_json_unittest.py

Change-Id: I629de3b5f98f34f09679e0b9c443acb4829905a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4621604
Commit-Queue: Xinan Lin <linxinan@chromium.org>
Reviewed-by: Sven Zheng <svenzheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1160838}