[serve] Move schema helpers out of api.py

[shrekris-anyscale]

Why are these changes needed?

api.py contains helper functions used internally to convert common data structures like Deployment to a schema and back. This causes cyclic dependencies and delayed import. This change moves (and in some cases removes) these helper functions to reduce delayed imports.

Functions moved:

• deployment_to_schema (to deployment.py)
• schema_to_deployment (to deployment.py)
• status_info_to_schema (to schema.py)
• serve_application_status_to_schema (to schema.py)

Functions removed:

• serve_application_to_schema
• schema_to_serve_application

Related issue number

N/A

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • This change relies on existing tests only.