feat(instance): add instance_servers data source

[Codelax]

This is an example of how plural datasources could be done and used.

It is used as

data scaleway_instance_servers "plural" {
  tags = ["mytag"]
}

resource scaleway_lb_backend "backend" {
  server_ips       = data.scaleway_instance_servers.plural.servers[*].public_ip
}

Currently public_ip is being put in schema and it would require to expand schema to expose more informations.
But we can still get all missing informations by using singular datasource after the plural one.

data scaleway_instance_servers "plural" {
  tags = ["yes"]
}

data scaleway_instance_server "singular" {
  for_each = {for server in data.scaleway_instance_servers.plural.servers: server.id => server}
  server_id = each.value.id
}

resource scaleway_lb_backend "backend" {
  server_ips       = values(data.scaleway_instance_server.singular)[*].public_ip
}

Using both datasource allow to get all informations while style filtering across all resources. It would be a great improvement while limiting the amount of code required.

[codecov-commenter]

Codecov Report

Merging #1264 (c1afba6) into master (3d6bf9c) will increase coverage by 0.12%.
The diff coverage is 82.02%.

@@            Coverage Diff             @@
##           master    #1264      +/-   ##
==========================================
+ Coverage   70.19%   70.31%   +0.12%     
==========================================
  Files         102      103       +1     
  Lines       16451    16629     +178     
==========================================
+ Hits        11547    11693     +146     
- Misses       3923     3947      +24     
- Partials      981      989       +8     

Impacted Files	Coverage Δ
scaleway/data_source_instance_servers.go	81.92% <81.92%> (ø)
scaleway/provider.go	73.39% <100.00%> (+0.11%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3d6bf9c...c1afba6. Read the comment docs.