refactor configurations

[wanghan-iapcm]

• new module conf is provided.
• create abstract base class conf.ConfGenerator
• all the configuration definitions are derived from the conf.ConfGenerator
• examples of conf.AlloyConfGenerator and conf.FileConfGenerator are provided. The former generate alloy configurations, and the latter generates confs from files.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.39252% with 6 lines in your changes missing coverage. Please review.

Project coverage is 80.94%. Comparing base (db4ea65) to head (fe3265d).
Report is 140 commits behind head on master.

Files with missing lines	Patch %	Lines
dpgen2/entrypoint/submit.py	33.33%	4 Missing ⚠️
dpgen2/conf/conf_generator.py	93.75%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   71.81%   80.94%   +9.12%     
==========================================
  Files          65       68       +3     
  Lines        2743     2829      +86     
==========================================
+ Hits         1970     2290     +320     
+ Misses        773      539     -234     

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