Add get/add/remove_derived_from to pystac.Item

[jsignell]

Related Issue(s):

• Add functionality to create STAC Items derived from other STAC Items #283

Description:

Add get_derived_from and set_derived_from to pystac.Item

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov]

Codecov Report

Patch coverage: 92.00% and no project coverage change.

Comparison is base (0829db8) 90.91% compared to head (f55f70c) 90.92%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1136   +/-   ##
=======================================
  Coverage   90.91%   90.92%           
=======================================
  Files          49       49           
  Lines        6606     6631   +25     
  Branches      970      975    +5     
=======================================
+ Hits         6006     6029   +23     
- Misses        421      422    +1     
- Partials      179      180    +1     

Impacted Files	Coverage Δ
pystac/item.py	93.33% <90.90%> (-0.27%)	⬇️
pystac/link.py	90.45% <100.00%> (+0.14%)	⬆️

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