Extend plan interfaces

[chhwang]

• Rename DefaultPlanner as Planner
• Add PlannerContext interface
• Add Plan Python class
• Add Architecture tag in the plan format
• Minor other interface changes

[codecov]

Codecov Report

Attention: Patch coverage is 94.46809% with 13 lines in your changes missing coverage. Please review.

Project coverage is 91.01%. Comparing base (dad034d) to head (5b29d86).

Files	Patch %	Lines
ark/model/model_json.cpp	61.53%	5 Missing ⚠️
ark/api/planner.cpp	98.33%	2 Missing ⚠️
ark/context_impl.cpp	91.30%	2 Missing ⚠️
ark/range.cpp	66.66%	2 Missing ⚠️
ark/api/executor.cpp	83.33%	1 Missing ⚠️
ark/model/model_graph_impl.cpp	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #231      +/-   ##
==========================================
+ Coverage   90.72%   91.01%   +0.29%     
==========================================
  Files         102      106       +4     
  Lines        3923     4106     +183     
==========================================
+ Hits         3559     3737     +178     
- Misses        364      369       +5     

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