Add ProjectionExtension.crs_string

[gadomski]

Related Issue(s): None

Description: This can be used to feed, e.g. rasterio's CRS.from_string without having to worry about which projection field is set.

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 #548 (17d94fb) into main (12eff70) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #548      +/-   ##
==========================================
+ Coverage   94.25%   94.27%   +0.02%     
==========================================
  Files          71       71              
  Lines       10263    10290      +27     
  Branches     1072     1077       +5     
==========================================
+ Hits         9673     9701      +28     
+ Misses        422      420       -2     
- Partials      168      169       +1     

Impacted Files	Coverage Δ
pystac/extensions/projection.py	98.49% <100.00%> (+0.12%)	⬆️
tests/extensions/test_projection.py	100.00% <100.00%> (ø)
pystac/extensions/base.py	92.45% <0.00%> (+1.88%)	⬆️

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[duckontheweb]

Looks good to me, thanks for adding this. Let's add a CHANGELOG entry for this as well and then it's good to merge.

[gadomski]

Done.