[Python API] Return job_id on sky.jobs.launch

[romilbhardwaj]

User report: it's hard to get job_id after submitting job with python API, requires correlating against jobs.queue and gets even harder is submitting concurrent jobs.

This PR returns job_id and handle from sky.jobs.launch().

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Manual python script

import sky

task = sky.Task(
    name='test',
    run='echo "Hello, World!"'
)

job_id = sky.jobs.launch(task=task)
print(job_id)

job_id = sky.jobs.launch(task=task)
print(job_id)

[Michaelvll]

Thanks @romilbhardwaj!