[chore] Change class name from process definition to workflow

[zhongjiajie]

Brief Summary of The Change

we should change the class keyword from process definition to workflow

fix: #22

Pull Request checklist

I confirm that the following checklist has been completed.

• Add/Change test cases for the changes.
• Add/Change the related documentation, should also change docs/source/config.rst when you change file default_config.yaml.
• (Optional) Add your change to UPDATING.md when it is an incompatible change.

[codecov-commenter]

Codecov Report

Merging #26 (8f7805c) into main (89f7407) will increase coverage by 0.02%.
The diff coverage is 85.07%.

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   90.65%   90.68%   +0.02%     
==========================================
  Files          57       59       +2     
  Lines        2055     2072      +17     
==========================================
+ Hits         1863     1879      +16     
- Misses        192      193       +1     

Flag	Coverage Δ
unittests	90.68% <85.07%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pydolphinscheduler/tasks/switch.py	100.00% <ø> (ø)
src/pydolphinscheduler/cli/commands.py	97.14% <50.00%> (ø)
src/pydolphinscheduler/core/workflow.py	81.86% <81.86%> (ø)
src/pydolphinscheduler/tasks/sub_workflow.py	83.33% <83.33%> (ø)
src/pydolphinscheduler/tasks/dependent.py	94.07% <87.50%> (-0.50%)	⬇️
src/pydolphinscheduler/core/task.py	93.67% <88.88%> (ø)
src/pydolphinscheduler/constants.py	100.00% <100.00%> (ø)
src/pydolphinscheduler/core/__init__.py	100.00% <100.00%> (ø)
src/pydolphinscheduler/core/process_definition.py	100.00% <100.00%> (+18.13%)	⬆️
src/pydolphinscheduler/core/yaml_workflow.py	91.82% <100.00%> (ø)
... and 3 more

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[zhongjiajie]

Should not merged until #15 merged, we need to change its keyword as well

[zhongjiajie]

ref: apache/dolphinscheduler#12918

[zhongjiajie]

ref: apache/dolphinscheduler#12918

released after apache/dolphinscheduler:3.1.2