feat(iam): add iam policy resource

[Codelax]

As this ressource needs an organization id, I added a mock one and replaced the one used in cassettes. This also assume there is a default project with the same id as the organization id one which should be the case. I'm open for alternative with the mocked organization id

[codecov-commenter]

Codecov Report

Merging #1344 (30a48e7) into master (4474c76) will decrease coverage by 1.37%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1344      +/-   ##
==========================================
- Coverage   69.81%   68.43%   -1.38%     
==========================================
  Files         109      113       +4     
  Lines       17646    18203     +557     
==========================================
+ Hits        12319    12457     +138     
- Misses       4262     4663     +401     
- Partials     1065     1083      +18     

Impacted Files	Coverage Δ
scaleway/helpers_iam.go	0.00% <0.00%> (ø)
scaleway/provider.go	72.09% <0.00%> (-1.03%)	⬇️
scaleway/resource_iam_policy.go	0.00% <0.00%> (ø)
scaleway/validation.go	54.05% <0.00%> (-20.03%)	⬇️
scaleway/resource_vpc_public_gateway.go	74.03% <0.00%> (-0.08%)	⬇️
scaleway/data_source_iam_user.go	0.00% <0.00%> (ø)
scaleway/resource_container_cron.go	66.66% <0.00%> (ø)
scaleway/data_source_iam_application.go	0.00% <0.00%> (ø)
scaleway/resource_container.go	61.09% <0.00%> (+3.15%)	⬆️
... and 1 more

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[remyleone]

Could you add documentation?