Add preserve_dict parameter to ItemCollection.from_dict

[lossyrob]

Related Issue(s): #453

Description:

When I made #454, I wasn't hip to the fact the ItemCollection got merged in already. This PR adds the preserve_dict parameter to ItemCollection.from_dict and uses preserve_dict=False when using ItemCollection.from_file.

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 #468 (2beaccd) into main (d51dd28) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #468   +/-   ##
=======================================
  Coverage   91.48%   91.48%           
=======================================
  Files          40       40           
  Lines        5240     5240           
=======================================
  Hits         4794     4794           
  Misses        446      446           

Impacted Files	Coverage Δ
pystac/item_collection.py	95.31% <100.00%> (ø)

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[duckontheweb]

Looks good. Thanks for adding this!