[feat] Aggregate polling for retrieving job pending reason

[vkarak]

This PR improves the polling of jobs for the pending reason.

This is now done in a single command for all pending jobs. Two knobs are also exposed to users now as configuration options and environment variables:

1. slurm_job_cancel_reasons: This is a list of pending reasons that reframe will check and will cancel the job proactively.
2. slurm_pending_job_reason_poll_freq: This controls the frequency that pending jobs will be polled for their pending reasons (valid only for slurm backend).

Closes #3640.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.66%. Comparing base (1eee4f9) to head (45c5613).

Files with missing lines	Patch %	Lines
reframe/core/schedulers/slurm.py	49.09%	28 Missing ⚠️
reframe/core/schedulers/__init__.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3642      +/-   ##
===========================================
- Coverage    91.70%   91.66%   -0.05%     
===========================================
  Files           62       62              
  Lines        13713    13723      +10     
===========================================
+ Hits         12576    12579       +3     
- Misses        1137     1144       +7     

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