feat(gateway_network): add vpc gateway network data source

[Codelax]

Fix #1382

[codecov-commenter]

Codecov Report

Merging #1383 (79d9a43) into master (b9b8015) will decrease coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #1383      +/-   ##
==========================================
- Coverage   66.38%   66.36%   -0.02%     
==========================================
  Files         123      124       +1     
  Lines       19624    19679      +55     
==========================================
+ Hits        13027    13060      +33     
- Misses       5460     5480      +20     
- Partials     1137     1139       +2     

Impacted Files	Coverage Δ
scaleway/data_source_vpc_gateway_network.go	59.25% <59.25%> (ø)
scaleway/provider.go	70.89% <100.00%> (+0.10%)	⬆️

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