Add metadata generator tool for test spec metadata generation.

Bug: 296873595
Test: Manual test (use go test inside tools/metadata/testdata)

Ignore-AOSP-First: CPing test_spec rule to udc-mainline-prod to support migration of test targets. Cherry pick of: aosp/2815214

Change-Id: I404b57224828149f26bcf4deadb662f513886231
Merged-In: I404b57224828149f26bcf4deadb662f513886231