Implement task generators and provisional nodes.

[tobiasraabe]

Closes #467.

• Rework DelayedPathNode to DirectoryNode if they are really the same?
• Fix typing delayed nodes. They do not need a save method for example
• Is Delayed the right name? Should it be Abstract or something like this
• How well do delayed nodes play along with the data catalog?
• Settle on name provisional or delayed or fuzzy? rename modules and variables accordingly.
• Decide whether the only way to generate tasks should be via @task to reduce the number of interfaces and maybe because it is the future?
• Should generator=True actually be is_generator=True.
• Should I find another abstraction for PNode and PProvisionalNode?

[codecov]

Codecov Report

Attention: Patch coverage is 97.35450% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 97.75%. Comparing base (ad3680e) to head (0d41ac6).
Report is 12 commits behind head on main.

❗ Current head 0d41ac6 differs from pull request most recent head 6e5679c. Consider uploading reports for the commit 6e5679c to get more accurate results

Files	Patch %	Lines
src/_pytask/delayed.py	88.88%	8 Missing ⚠️
src/_pytask/delayed_utils.py	96.96%	1 Missing ⚠️
src/_pytask/shared.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #487      +/-   ##
==========================================
- Coverage   97.80%   97.75%   -0.06%     
==========================================
  Files         106      108       +2     
  Lines        8710     9042     +332     
==========================================
+ Hits         8519     8839     +320     
- Misses        191      203      +12     

Flag	Coverage Δ
end_to_end	85.02% <94.70%> (+2.75%)	⬆️
integration	40.83% <49.47%> (+0.33%)	⬆️
unit	64.98% <53.70%> (-4.71%)	⬇️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tobiasraabe]

Hi @janosg, @timmens, do you have time for some feedback? Could you read the how-to guide and flag anything about the feature that comes to your mind? Don't worry if you do not have time. Thanks a lot!