[SkyServe] Add service schema and change to new service YAML

[cblmemo]

This PR adds a schema checking for our newer version service YAML, and change to new YAML config pattern.

An example:

service:
  port: 8082
  readiness_probe:
    path: /health
    timeout: 1200
  replica_policy:
    min_replica: 1
    max_replica: 5             # optional
    qpm_upper_threshold: 10    # optional, if qpm is higher, scale up
    qpm_lower_threshold: 1     # optional, if qpm is lower, scale down

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• 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

[infwinston]

Thanks @cblmemo ! The code looks great and I left a few comments.

[infwinston]

what if self.upper_threshold is None, then we shouldn't run the below code right?
we probably should refactor the code a bit to incorporate fix-node policy and autoscaling policy.

[cblmemo]

Sure.. I'll just check if it is None for now, then if user only specify upper_threshold, that means user only want to autoscale up and #replica will only increase until reach max_replica; vise versa for only specify lower_threshold case. Only if user specified both upper and lower threshold will enable auto sale up/down. We might need a discussion on here.

[cblmemo]

Close and move to #2276.