Add Label Extension summaries

[duckontheweb]

Related Issue(s):

• Implement collection summary functionality #327

Description:

• Defines equality for component classes in the Label Extension (e.g. LabelClasses, LabelProperties, etc.)
• Adds summaries for fields defined in stac-fields
• Uses Enum for "label:tasks", but continues to allow free string input (following pattern for RelType).

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.

[codecov-commenter]

Codecov Report

Merging #484 (4997826) into main (f742fa4) will increase coverage by 0.04%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##             main     #484      +/-   ##
==========================================
+ Coverage   91.66%   91.71%   +0.04%     
==========================================
  Files          40       40              
  Lines        5245     5310      +65     
==========================================
+ Hits         4808     4870      +62     
- Misses        437      440       +3     

Impacted Files	Coverage Δ
pystac/extensions/label.py	96.30% <95.65%> (-0.21%)	⬇️

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