Add Singleton class to collect and validate test spec metadata.

Bug: 296873595
Test: Manual testing (Will add unit test case in the next change.)

Ignore-AOSP-First: CPing test_spec rule to udc-mainline-prod to support migration of test targets. Cherry pick of:aosp/2824671

Change-Id: Ic4177c5f76602088d52a31ca8d9fbaa703855837
Merged-In: Ic4177c5f76602088d52a31ca8d9fbaa703855837