Avoid calling deepcopy in from_dict methods when unnecessary

[lossyrob]

Related Issue(s): #453

Description:
This PR adds a parameter to the STACObject.from_dict method and implementations named preserve_dict. If this is True, the dict that is passed in as a parameter will not be mutated during the operation - which is achieved by making a deep copy of dict parameter in the implementing methods. If preserve_dict is False, the incoming dict is mutated and a deepcopy is avoided. The default of this parameter is True, so that users can avoid a situation where their parameters are mutated unsuspectedly. In situations where the dict is a transient encoding of the STAC object, users should use preserve_dict=False to avoid incurring the performance cost of the deepcopy.

preserve_dict is set to False for methods that read a STAC object from a file, as there's no need to avoid mutating the dict in those cases.

This should help address some of the performance issues in described in stac-utils/pystac-client#49, as pystac-client will be able to use preserve_dict=False in the ItemCollection.from_dict method

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.

[duckontheweb]

👍 , thanks for getting this in. Just 1 minor docstring change and a possible change to some assertions that you can take or leave.

[codecov-commenter]

Codecov Report

Merging #454 (68df9bd) into main (2739661) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #454   +/-   ##
=======================================
  Coverage   91.48%   91.48%           
=======================================
  Files          40       40           
  Lines        5237     5240    +3     
=======================================
+ Hits         4791     4794    +3     
  Misses        446      446           

Impacted Files	Coverage Δ
pystac/catalog.py	94.72% <100.00%> (+0.01%)	⬆️
pystac/collection.py	95.51% <100.00%> (+0.01%)	⬆️
pystac/item.py	96.99% <100.00%> (+<0.01%)	⬆️
pystac/stac_io.py	77.19% <100.00%> (ø)
pystac/stac_object.py	94.70% <100.00%> (ø)

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