Add projection field for charge densities

[munrojm]

Adds s3_prefix_url as a projected field to the charge density object API endpoint.

[codecov-commenter]

Codecov Report

Merging #397 (646b9d6) into main (86f3035) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #397   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files          87       87           
  Lines        4686     4686           
=======================================
  Hits         4329     4329           
  Misses        357      357           

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