Add MGRS Extension from code in stactools-package for sentinel2

[jsignell]

Related Issue(s):

• Create MGRS Extension from code in stactools-package for sentinel2 #778

Description:

Code from: https://github.com/stactools-packages/sentinel2/blob/main/src/stactools/sentinel2/mgrs.py
Example from: https://github.com/stac-extensions/mgrs/blob/main/examples/item.json

Added pytest-style tests, docs

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-commenter]

Codecov Report

Patch coverage: 96.10% and project coverage change: +0.07 🎉

Comparison is base (1fa72d0) 90.23% compared to head (71bf305) 90.30%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1088      +/-   ##
==========================================
+ Coverage   90.23%   90.30%   +0.07%     
==========================================
  Files          47       48       +1     
  Lines        6236     6313      +77     
  Branches      936      943       +7     
==========================================
+ Hits         5627     5701      +74     
- Misses        428      430       +2     
- Partials      181      182       +1     

Impacted Files	Coverage Δ
pystac/extensions/mgrs.py	96.05% <96.05%> (ø)
pystac/__init__.py	94.11% <100.00%> (+0.11%)	⬆️

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[philvarner]

Thanks for doing this @jsignell