[Core] Fix for sky status --endpoints when a range of ports is opened

[romilbhardwaj]

When multiple ports are opened, sky status --endpoint 8888 <cluster> would fail. This is because the handle.launched_resources.ports contains a str like '8888-8889'. We need to convert to a set of ints and then compare in our code.

Repro:

resources:
  cloud: kubernetes
  ports:
    - 8888
    - 8889

run: |
    python -m http.server 8888 & python -m http.server 8889;

# Before:
$ sky status --endpoint 8889 mycluster
ValueError: Port 8889 is not exposed on cluster 'test'.

# After fix:
$ sky status --endpoint 8889 mycluster
34.67.101.159:8889

FYI - code paths for endpoints are refactored out of cli.py in #3109.

• Code formatting: bash format.sh
• Tested manually for the above repro with a) no ports, b)1 port c) many ports

[Michaelvll]

Great catch @romilbhardwaj! LGTM.