Auto-build/update codegen

Exempt-From-Owner-Approval: introduces separate utility; doesn't touch anything around it
Test: 1. Manually enter version/presence of build artifact,
and ensure `codegen FILENAME` still works
2. run codegen again without altering version, ensure no build triggered
Change-Id: Iaa253a430e627d0e7c25781cde535958f77db97f
2 files changed