[Serve] Support customizable readiness probe timeout

[cblmemo]

Expose the readiness probe timeout to users. Example:

# test.yaml
service:
  readiness_probe:
    path: /health
    initial_delay_seconds: 20
    timeout_seconds: 100
  replicas: 1

resources:
  cpus: 2+
  ports: 8081

run: python -m http.server 8081

$ sky serve up test.yaml
Service from YAML spec: test.yaml
Service Spec:
Readiness probe method:           GET /health
Readiness initial delay seconds:  20
Readiness probe timeout seconds:  100
Replica autoscaling policy:       Fixed 1 replica
Spot Policy:                      No spot policy

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • The YAML above
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[Michaelvll]

Should we use the timeout for the post request directly or should we instead handle the timeout ourselves for when to set the replica to be NOT_READY? Reason: changing the timeout for the post request will cause the probing blocked by the a single request.

[cblmemo]

Could you elaborate on handle the timeout ourselves for when to set the replica to be NOT_READY..? Do you refer to letting the user configure the consecutive failure timeout?

[cblmemo]

Though it is from a user's request

[Michaelvll]

I mean should we have the user to customize _CONSECUTIVE_FAILURE_THRESHOLD_TIMEOUT, instead of changing the timeout?