Add go.mod to example/celler

[akojo]

Describe the PR
Since example/celler now depends on modules that main go.mod doesn't, running go mod tidy adds unnecessary dependencies to main go.mod. Fix by adding go.mod to example/celler

Relation issue
Related to e.g. #826 and #563

Additional context
Fixes an issue created by #800

[codecov]

Codecov Report

Merging #838 (c975eae) into master (3d083d3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #838   +/-   ##
=======================================
  Coverage   84.88%   84.88%           
=======================================
  Files           8        8           
  Lines        1654     1654           
=======================================
  Hits         1404     1404           
  Misses        145      145           
  Partials      105      105           

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