Onboard public_ip_associate resource (#582)

* onboard public_ip_associate resource

* add warnings

* add example

* edit warning message

* Update stackit/internal/services/iaas/publicipassociate/resource.go

Co-authored-by: João Palet <joao.palet@outlook.com>

* Update stackit/internal/services/iaas/publicipassociate/resource.go

Co-authored-by: João Palet <joao.palet@outlook.com>

* Update stackit/internal/services/iaas/publicipassociate/resource.go

Co-authored-by: João Palet <joao.palet@outlook.com>

* Update stackit/internal/services/iaas/publicipassociate/resource.go

Co-authored-by: João Palet <joao.palet@outlook.com>

* Update stackit/internal/services/iaas/publicipassociate/resource.go

Co-authored-by: João Palet <joao.palet@outlook.com>

* Update stackit/internal/services/iaas/publicipassociate/resource.go

Co-authored-by: João Palet <joao.palet@outlook.com>

* Update stackit/internal/services/iaas/publicipassociate/resource.go

Co-authored-by: João Palet <joao.palet@outlook.com>

* Update stackit/internal/services/iaas/publicipassociate/resource.go

Co-authored-by: João Palet <joao.palet@outlook.com>

* add requiresReplace in network_interface_id

* change function name to toCreatePayload

* change deletion of the public ip association

* add unit test

* Add missing space between sentences

* update conflict warning message

* update conflict warning message

* Adjust warning message

---------

Co-authored-by: João Palet <joao.palet@outlook.com>
Co-authored-by: João Palet <joao.palet@freiheit.com>

docs/resources/public_ip_associate.md

@@ -0,0 +1,41 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_public_ip_associate Resource - stackit"
+subcategory: ""
+description: |-
+  Associates an existing public IP to a network interface. This is useful for situations where you have a pre-allocated public IP or unable to use the stackit_public_ip resource to create a new public IP. Must have a region specified in the provider configuration.
+  !> The stackit_public_ip_associate resource should not be used together with the stackit_public_ip resource for the same network interface, as they both have control of the network interface association and this will lead to conflicts.
+  ~> This resource is in beta and may be subject to breaking changes in the future. Use with caution. See our guide https://registry.terraform.io/providers/stackitcloud/stackit/latest/docs/guides/opting_into_beta_resources for how to opt-in to use beta resources.
+---
+
+# stackit_public_ip_associate (Resource)
+
+Associates an existing public IP to a network interface. This is useful for situations where you have a pre-allocated public IP or unable to use the `stackit_public_ip` resource to create a new public IP. Must have a `region` specified in the provider configuration.
+
+!> The `stackit_public_ip_associate` resource should not be used together with the `stackit_public_ip` resource for the same network interface, as they both have control of the network interface association and this will lead to conflicts.
+
+~> This resource is in beta and may be subject to breaking changes in the future. Use with caution. See our [guide](https://registry.terraform.io/providers/stackitcloud/stackit/latest/docs/guides/opting_into_beta_resources) for how to opt-in to use beta resources.
+
+## Example Usage
+
+```terraform
+resource "stackit_public_ip_associate" "example" {
+  project_id           = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  public_ip_id         = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  network_interface_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `network_interface_id` (String) The ID of the network interface (or virtual IP) to which the public IP should be attached to.
+- `project_id` (String) STACKIT project ID to which the public IP is associated.
+- `public_ip_id` (String) The public IP ID.
+
+### Read-Only
+
+- `id` (String) Terraform's internal resource ID. It is structured as "`project_id`,`public_ip_id`,`network_interface_id`".
+- `ip` (String) The IP address.