Register /readyz in ate-api and wait for components to be ready

[Benjamin Elder (BenTheElder)]

• Add a trivial /readyz handler (which will only be served once we finish initializing)
  • We use the metrics handler because we already expose an HTTP port there and there should be no conflict
• Add a readiness probe to the install manifest
• Update install script to wait for readiness on core components

Improves reliability for the demo flow, including the test flake seen in #2

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to documentation are included in the PR