Job context implementation

[samplise]

What changes were proposed in this pull request?

Wrap up job context into the object JobContext.

Why are the changes needed?

Ensure the job context is accessed with control(Avoid implicit object update operations whenever possible ).

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Unit test

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.86358% with 73 lines in your changes missing coverage. Please review.

Project coverage is 80.65%. Comparing base (de6c5b1) to head (76a6f3a). Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
dlrover/python/master/node/dist_job_manager.py	78.49%	20 Missing :warning:
dlrover/python/master/node/worker.py	62.16%	14 Missing :warning:
dlrover/python/master/node/training_node.py	81.57%	7 Missing :warning:
...lrover/python/diagnosis/common/diagnosis_action.py	94.28%	6 Missing :warning:
dlrover/python/elastic_agent/torch/training.py	28.57%	5 Missing :warning:
dlrover/python/master/node/job_context.py	92.53%	5 Missing :warning:
dlrover/python/master/node/local_job_manager.py	79.16%	5 Missing :warning:
dlrover/python/master/node/ps.py	84.61%	4 Missing :warning:
dlrover/python/elastic_agent/master_client.py	83.33%	2 Missing :warning:
dlrover/python/elastic_agent/monitor/training.py	33.33%	2 Missing :warning:
... and 3 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1319 +/- ## ========================================== + Coverage 80.46% 80.65% +0.19% ========================================== Files 222 225 +3 Lines 20740 21262 +522 ========================================== + Hits 16688 17149 +461 - Misses 4052 4113 +61 ```

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