feat(iam): add iam user resource

[gbloquel]

Implements the resource iam_user

This PR closes #2088

[gbloquel]

Hi @Codelax, @Monitob

Could you review my PR ?
I forgot to put a reviewer and as the workflow requires one.

regards,

[Codelax]

Hello, thanks for your contribution.
I will keep your PR in mind and will not forget.
It is fine as you did, I will merge once I can spend more time on it.
Before releasing it we will review the features with the IAM team to be sure this is the best interface we can propose and check this is future proof.

[codecov-commenter]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (5759d40) 73.19% compared to head (a91dccf) 73.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2152      +/-   ##
==========================================
+ Coverage   73.19%   73.25%   +0.05%     
==========================================
  Files         202      203       +1     
  Lines       32042    32152     +110     
==========================================
+ Hits        23454    23552      +98     
- Misses       6564     6573       +9     
- Partials     2024     2027       +3     

Files	Coverage Δ
scaleway/provider.go	80.50% <100.00%> (+0.05%)	⬆️
scaleway/resource_iam_user.go	88.99% <88.99%> (ø)

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