Replace deprecated MemoryResource -> ResourceSharedPtr

[rhaschke]

Addresses ros/resource_retriever@a12b2b8
Fixes #260

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.06%. Comparing base (e733cb1) to head (eb32275).
Report is 3 commits behind head on ros2.

Additional details and impacted files

@@           Coverage Diff           @@
##             ros2     #263   +/-   ##
=======================================
  Coverage   69.06%   69.06%           
=======================================
  Files          21       21           
  Lines        3370     3370           
  Branches      325      325           
=======================================
  Hits         2327     2327           
  Misses        932      932           
  Partials      111      111           

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