Add non-blocking get method

[sschlenkrich]

This PR adds the build_async operation to the GET method.

If the requested object is a Future and if it is not (yet) ready then the server returns an error instead of waiting for the object being calculated.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.58%. Comparing base (446c8f7) to head (7debe35).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   90.43%   90.58%   +0.14%     
==========================================
  Files          10       10              
  Lines         251      255       +4     
==========================================
+ Hits          227      231       +4     
  Misses         24       24              

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