Preserve Collection assets on clone

[duckontheweb]

Related Issue(s):

• Closes Collection assets removed on clone() #747

Description:

Adds an assets argument to Item.__init__ and Collection.__init__ that accepts a dictionary mapping strings to Asset objects, which allows us to attach Assets to these objects as part of instantiation.

Also preserves Collection assets when using Collection.clone and adds regression test.

cc: @cholden-ag

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 #834 (a2febda) into main (0c6074a) will decrease coverage by 0.00%.
The diff coverage is 93.93%.

@@            Coverage Diff             @@
##             main     #834      +/-   ##
==========================================
- Coverage   94.34%   94.33%   -0.01%     
==========================================
  Files          83       83              
  Lines       11923    11939      +16     
  Branches     1390     1400      +10     
==========================================
+ Hits        11249    11263      +14     
  Misses        495      495              
- Partials      179      181       +2     

Impacted Files	Coverage Δ
tests/test_item.py	99.54% <91.66%> (-0.46%)	⬇️
tests/test_collection.py	99.56% <92.30%> (-0.44%)	⬇️
pystac/collection.py	92.46% <100.00%> (ø)
pystac/item.py	88.77% <100.00%> (-0.12%)	⬇️

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[cholden-ag]

Sorry to leave the PR to my issue hanging @duckontheweb and thanks for adding the fix. I think this looks good for fixing the clone() behavior on Collections, and having assets in the constructor will also improve the user experience for creating some Items/Collections since it'll take care of running add_asset