Implement new stackit_network resource and datasource (#366)

* Copy file from DNS zone resource

* Implement resource

* Implement datasource

* Add examples

* Implement acceptance test; Some fixes

* Add docs

* Fix linter

* Fixes after review

docs/data-sources/network.md

@@ -0,0 +1,37 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_network Data Source - stackit"
+subcategory: ""
+description: |-
+  Network resource schema. Must have a region specified in the provider configuration.
+---
+
+# stackit_network (Data Source)
+
+Network resource schema. Must have a `region` specified in the provider configuration.
+
+## Example Usage
+
+```terraform
+data "stackit_network" "example" {
+  project_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  network_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `network_id` (String) The network ID.
+- `project_id` (String) STACKIT project ID to which the network is associated.
+
+### Read-Only
+
+- `id` (String) Terraform's internal resource ID. It is structured as "`project_id`,`network_id`".
+- `ipv4_prefix_length` (Number) The IPv4 prefix length of the network.
+- `name` (String) The name of the network.
+- `nameservers` (List of String) The nameservers of the network.
+- `prefixes` (List of String) The prefixes of the network.
+- `public_ip` (String) The public IP of the network.