chore(terraform): use a util func to build internal terraform id

[rubenhoenle]

Description

Instead of duplicating the code everywhere to build the internal terraform id for resource/datasource implementations, it's now a util func which can be called in a single line. Makes it much more easy to read IMO.

// BEFORE
idParts := []string{
  model.ProjectId.ValueString(),
  model.BarId.ValueString(),
}
  model.Id = types.StringValue(strings.Join(idParts, core.Separator),
)

// AFTER
model.Id = utils.BuildInternalTerraformId(model.ProjectId.ValueString(), model.BarId.ValueString())

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[marceljk]

Looks good and makes it more readable!
I just found a few resources where the ID build can be adjusted:

terraform-provider-stackit/stackit/internal/services/cdn/customdomain/resource.go

Lines 277 to 279 in 5d844a8

	id := model.ProjectId.ValueString() + core.Separator + model.DistributionId.ValueString() + core.Separator + *customDomain.Name
	model.ID = types.StringValue(id)

terraform-provider-stackit/stackit/internal/services/cdn/distribution/resource.go

Lines 450 to 452 in 7ce8027

	id := *distribution.ProjectId + core.Separator + *distribution.Id
	model.ID = types.StringValue(id)

terraform-provider-stackit/stackit/internal/services/iaas/publicipranges/datasource.go

Lines 178 to 179 in 53ec994

	modelId := strings.Join(apiIpRanges, ",")
	model.Id = types.StringValue(modelId)