add subworkflow tasks to workflows

[silvavelosa]

Description

Add support for subworkflow tasks

Checklist:

• Tests added/updated.
• Documentation updated. Documentation is generated from docstrings - these must be updated according to your change.
  If a new method has been added it should be referenced in cognite.rst in order to generate docs based on its docstring.
• Changelog updated in CHANGELOG.md.
• Version bumped. If triggering a new release is desired, bump the version number in _version.py and pyproject.toml per semantic versioning.

[codecov]

Codecov Report

Merging #1445 (4e2b88d) into master (8fec922) will decrease coverage by 0.06%.
The diff coverage is 79.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1445      +/-   ##
==========================================
- Coverage   91.80%   91.74%   -0.06%     
==========================================
  Files         120      120              
  Lines       15599    15623      +24     
==========================================
+ Hits        14321    14334      +13     
- Misses       1278     1289      +11     

Files	Coverage Δ
cognite/client/_version.py	100.00% <100.00%> (ø)
cognite/client/data_classes/__init__.py	100.00% <ø> (ø)
cognite/client/data_classes/workflows.py	94.65% <78.57%> (-1.07%)	⬇️

... and 2 files with indirect coverage changes

[doctrino]

Just a few minor changes.