[feat] Extend the public API of Job

[vkarak]

This PR exposes several Job attributes publicly by practically documenting them. It also uses the variable builtin to define them and also refactors a bit the arguments passed to the constructor of a Job. Now, the arguments that are passed to it are only those that must be set from the frontend either directly or indirectly through the configuration.

Fixes #2315.

[codecov-commenter]

Codecov Report

Merging #2463 (3449656) into master (7b27cbe) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2463   +/-   ##
=======================================
  Coverage   85.71%   85.71%           
=======================================
  Files          56       56           
  Lines       10536    10529    -7     
=======================================
- Hits         9031     9025    -6     
+ Misses       1505     1504    -1     

Impacted Files	Coverage Δ
reframe/core/launchers/mpi.py	98.90% <100.00%> (ø)
reframe/core/pipeline.py	93.38% <100.00%> (+0.01%)	⬆️
reframe/core/schedulers/__init__.py	98.28% <100.00%> (+0.45%)	⬆️
reframe/core/schedulers/lsf.py	48.75% <100.00%> (ø)
reframe/core/schedulers/slurm.py	52.30% <100.00%> (ø)

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