Summary: Encapsulate all top level messages between vizier and cloud.

Test Plan: N/A

Reviewers: michelle, philkuz, #engineering

Reviewed By: michelle, #engineering

Differential Revision: https://phab.corp.pixielabs.ai/D3696

GitOrigin-RevId: b5bedc840b4663a7b2c7f6dd69c1e454e03a1951