feat(iam): add iam user datasource

[yfodil]

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[codecov-commenter]

Codecov Report

Merging #1356 (ecdd826) into master (b693d30) will decrease coverage by 0.31%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1356      +/-   ##
==========================================
- Coverage   69.66%   69.34%   -0.32%     
==========================================
  Files         111      112       +1     
  Lines       17871    17953      +82     
==========================================
  Hits        12449    12449              
- Misses       4340     4422      +82     
  Partials     1082     1082              

Impacted Files	Coverage Δ
scaleway/data_source_iam_user.go	0.00% <0.00%> (ø)
scaleway/provider.go	72.37% <0.00%> (-0.29%)	⬇️
scaleway/validation.go	54.05% <0.00%> (-20.03%)	⬇️

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