Add variables setter to datacube extension

[duckontheweb]

Related Issue(s):

• Closes DatacubeExtension.variables doesn't have a setter #681
• Begins to address Tests and documentation for Datacube extension #324

Description:

Adds a setter for the DatacubeExtension.variables property and adds a Variable.to_dict method. Also adds tests for getting and setting DatacubeExtension.variables.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• 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.

[duckontheweb]

@TomAugspurger it would be great to get your review on this as well.

[codecov-commenter]

Codecov Report

Merging #699 (cda8a98) into main (f68cf81) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #699      +/-   ##
==========================================
+ Coverage   94.20%   94.25%   +0.04%     
==========================================
  Files          77       77              
  Lines       11120    11140      +20     
  Branches     1337     1339       +2     
==========================================
+ Hits        10476    10500      +24     
+ Misses        466      461       -5     
- Partials      178      179       +1     

Impacted Files	Coverage Δ
pystac/extensions/datacube.py	65.67% <100.00%> (+1.73%)	⬆️
tests/extensions/test_datacube.py	100.00% <100.00%> (ø)

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