Fix serialization of FeedScopedId in geocoder sandbox

[leonardehrenfried]

Summary

When we removed the REST API in #6578 we missed that the geocoder sandbox feature, used by Arcadis, had a dependency on the custom serializer for feed-scoped ids. This meant that the output for IDs in the geocoder endpoint changed.

This PR restores the old behaviour by adding a custom serializer to the sandbox code.

Issue

Not issue, since this is a sandbox feature.

Unit tests

Added.

cc @miles-grant-ibigroup

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.31%. Comparing base (2513946) to head (d6bc473).
Report is 16 commits behind head on dev-2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #6671   +/-   ##
==========================================
  Coverage      71.31%   71.31%           
+ Complexity     18518    18517    -1     
==========================================
  Files           2031     2031           
  Lines          76460    76462    +2     
  Branches        7820     7820           
==========================================
+ Hits           54524    54526    +2     
  Misses         19165    19165           
  Partials        2771     2771           

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[leonardehrenfried]

This only touches the sandbox so I will merge with one reviewer.