Return calling class from Link constructor methods

[duckontheweb]

Related Issue(s):

Description:

Following in the footsteps of #451, this changes the Link constructor methods (Link.from_dict, etc.) to return the calling class instead of hard-coding the Link class as the return type. This allows downstream libraries and users to sub-class Link and still be able to use these constructors.

UPDATE: Also does the same for Asset constructors and for clone methods on Catalog, Collection, Item, Link, and Asset classes.

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 #512 (44157ab) into main (a4fe4cd) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #512      +/-   ##
==========================================
+ Coverage   93.96%   93.97%   +0.01%     
==========================================
  Files          71       71              
  Lines        9991    10013      +22     
  Branches     1061     1061              
==========================================
+ Hits         9388     9410      +22     
  Misses        434      434              
  Partials      169      169              

Impacted Files	Coverage Δ
pystac/link.py	92.18% <100.00%> (ø)
tests/test_link.py	100.00% <100.00%> (ø)

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