Refactor engine

[superstar54]

This PR splits the WorkGraphEngine into five parts:

• WorkGraphEngine
• ContextManager
• AwaitableManager
• TaskManager
• ErrorHandlerManager

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.48420% with 99 lines in your changes missing coverage. Please review.

Project coverage is 81.67%. Comparing base (5937b88) to head (3be40c6).
Report is 102 commits behind head on main.

Files with missing lines	Patch %	Lines
aiida_workgraph/engine/task_manager.py	89.04%	64 Missing ⚠️
aiida_workgraph/engine/awaitable_manager.py	82.52%	18 Missing ⚠️
aiida_workgraph/engine/context_manager.py	76.47%	8 Missing ⚠️
aiida_workgraph/engine/workgraph.py	79.31%	6 Missing ⚠️
aiida_workgraph/engine/error_handler_manager.py	92.68%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #367      +/-   ##
==========================================
+ Coverage   75.75%   81.67%   +5.91%     
==========================================
  Files          70       65       -5     
  Lines        4615     4872     +257     
==========================================
+ Hits         3496     3979     +483     
+ Misses       1119      893     -226     

Flag	Coverage Δ
python-3.11	81.60% <87.48%> (+5.94%)	⬆️
python-3.12	81.60% <87.48%> (?)
python-3.9	81.66% <87.48%> (+5.92%)	⬆️

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