feat!: Rename backend -> platform-plugin-sample

[kdmccormick]

Why?

• platform-plugin- is a standard backend plugin prefix which Tutor recognizes.
• By including the word sample, it's more obvious for users of this repo how to adapt it into their own backend plugin (by replacing all occurances of "sample")

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.40%. Comparing base (7d753b3) to head (e539391).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #35   +/-   ##
=======================================
  Coverage   88.40%   88.40%           
=======================================
  Files          14       14           
  Lines         414      414           
  Branches       16       16           
=======================================
  Hits          366      366           
  Misses         40       40           
  Partials        8        8           

Flag	Coverage Δ
unittests	88.40% <ø> (ø)

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