alternate-assets extension + tests

[emmanuelmathot]

Description:

Manage Alternate-Assets extension with rapid accessor for assets objects

PR Checklist:

• Tests pass (run dotnet test src).
• This PR is made against develop branch.
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG.

[codecov-commenter]

Codecov Report

Merging #13 (c0899e0) into develop (9db122a) will increase coverage by 0.28%.
The diff coverage is 83.72%.

@@             Coverage Diff             @@
##           develop      #13      +/-   ##
===========================================
+ Coverage    68.47%   68.76%   +0.28%     
===========================================
  Files           59       61       +2     
  Lines         2233     2276      +43     
  Branches       190      192       +2     
===========================================
+ Hits          1529     1565      +36     
- Misses         704      711       +7     

Flag	Coverage Δ
unittests	66.82% <83.72%> (+0.32%)	⬆️

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Impacted Files	Coverage Δ
...tac/Extensions/Alternate/AlternateStacExtension.cs	80.00% <80.00%> (ø)
...tStac/Extensions/Alternate/AlternateAssetObject.cs	92.30% <92.30%> (ø)

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