Add an optional function source to workflow recipes

[liamhuber]

Follows the same format as the atomic node fully qualified name, and centralizes the shared behaviour.

For workflows, this is optional, since they may be being dynamically created, e.g. as the bodies of flow controls, or just written directly as JSON to be later parsed. Still, in the case they come from parsed python it doesn't hurt, and I hope it will be useful when we get to integration with semantikon.

Closes #131

[github-actions]

👈 Launch a binder notebook on branch pyiron/flowrep/workflow-source

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.94%. Comparing base (ddeba61) to head (80e6052).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   98.93%   98.94%           
=======================================
  Files          27       27           
  Lines        1882     1888    +6     
=======================================
+ Hits         1862     1868    +6     
  Misses         20       20           

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