[SERVE] Add headers to the readiness probe

[JGSweets]

If the serve function deploys a feature which utilizes authentication via headers, the readiness probe cannot interact with it.

Solution:

• update schema for readiness_probe to include a headers dictionary
• update sky/serve/replica_managers.py::ReplicaInfo to utilize headers when making the request.
• (optional) allow the schema to utilize the envs section to utilize values in the service

[Michaelvll]

This should be added in #3552, and we are adding the doc for it #3587. Would you like to have a try for SkyPilot’s nightly build with pip uninstall skypilot; pip install -U skypilot-nightly

envs:
  AUTH_TOKEN: # left empty as a required filled during launch
services:
  readiness_probe:
    path: /v1/models
    # Set authorization headers here if needed.
    headers:
      Authorization: Bearer $AUTH_TOKEN

[JGSweets]

Wicked! I missed that.