resource/gitlab_application_settings: New experimental resource

This change set implements the new `gitlab_application_settings`
resource which provides a way to change the GitLab Application Settings
using the API at
https://docs.gitlab.com/ee/api/settings.html#change-application-settings.

Even though I suggest that we can merge this first experimental version
of the resource there are still quite some open questions for me.

I'd like to tackle them over at
https://github.com/gitlabhq/terraform-provider-gitlab/issues/957 and
give the community the change to take part in the discussion on how to
move forward with this.

Refs: #957 #1187

examples/resources/gitlab_application_settings/resource.tf

@@ -0,0 +1,10 @@
+# Set the default branch
+resource "gitlab_application_settings" "this" {
+  default_branch_name = "main"
+}
+
+# Set the 2FA settings
+resource "gitlab_application_settings" "this" {
+  require_two_factor_authentication = true
+  two_factor_grace_period           = 24
+}

internal/provider/resource_gitlab_application_settings.go

@@ -0,0 +1,78 @@
+package provider
+
+import (
+	"context"
+	"log"
+
+	"github.com/hashicorp/terraform-plugin-sdk/v2/diag"
+	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
+	gitlab "github.com/xanzy/go-gitlab"
+)
+
+const applicationSettingsID = "gitlab"
+
+var _ = registerResource("gitlab_application_settings", func() *schema.Resource {
+	return &schema.Resource{
+		Description: `The ` + "`" + `gitlab_application_settings` + "`" + ` resource allows to manage the GitLabLab application settings.
+
+~> This is an **experimental resource**. By nature it doesn't properly fit into how Terraform resources are meant to work.
+   Feel free to join the [discussion](https://github.com/gitlabhq/terraform-provider-gitlab/issues/957) if you have any
+   ideas or questions regarding this resource.
+
+~> All ` + "`" + `gitlab_application_settings` + "`" + ` use the same ID ` + "`" + `gitlab` + "`" + `.
+
+!> This resource does not implement any destroy logic, it's a no-op at this point.
+   It's also not possible to revert to the previous settings.
+
+-> Requires at administrative privileges on GitLab.
+
+**Upstream API**: [GitLab REST API docs](https://docs.gitlab.com/ee/api/settings.html)`,
+
+		CreateContext: resourceGitlabApplicationSettingsSet,
+		ReadContext:   resourceGitlabApplicationSettingsRead,
+		UpdateContext: resourceGitlabApplicationSettingsSet,
+		DeleteContext: resourceGitlabApplicationSettingsDelete,
+
+		Schema: gitlabApplicationSettingsSchema(),
+	}
+})
+
+func resourceGitlabApplicationSettingsSet(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
+	client := meta.(*gitlab.Client)
+
+	log.Printf("[DEBUG] update GitLab Application Settings")
+	options := gitlabApplicationSettingsToUpdateOptions(d)
+	if (gitlab.UpdateSettingsOptions{}) != *options {
+		_, _, err := client.Settings.UpdateSettings(options, gitlab.WithContext(ctx))
+		if err != nil {
+			return diag.FromErr(err)
+		}
+	}
+
+	d.SetId(applicationSettingsID)
+	return resourceGitlabApplicationSettingsRead(ctx, d, meta)
+}
+
+func resourceGitlabApplicationSettingsRead(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
+	if d.Id() != applicationSettingsID {
+		return diag.Errorf("The `gitlab_application_settings` resource can only exist once and requires the id to be `gitlab`")
+	}
+
+	client := meta.(*gitlab.Client)
+	log.Printf("[DEBUG] read GitLab Application settings")
+	settings, _, err := client.Settings.GetSettings(gitlab.WithContext(ctx))
+	if err != nil {
+		return diag.FromErr(err)
+	}
+
+	stateMap := gitlabApplicationSettingsToStateMap(settings)
+	if err = setStateMapInResourceData(stateMap, d); err != nil {
+		return diag.FromErr(err)
+	}
+	return nil
+}
+
+func resourceGitlabApplicationSettingsDelete(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
+	log.Printf("[DEBUG] destroying the application settings does not yet do anything.")
+	return nil
+}

internal/provider/resource_gitlab_application_settings_test.go

@@ -0,0 +1,33 @@
+//go:build acceptance
+// +build acceptance
+
+package provider
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource"
+)
+
+func TestAccGitlabApplicationSettings_basic(t *testing.T) {
+	// lintignore:AT001
+	resource.Test(t, resource.TestCase{
+		ProviderFactories: providerFactories,
+		Steps: []resource.TestStep{
+			// Verify empty application settings
+			{
+				Config: `
+					resource "gitlab_application_settings" "this" {}
+				`,
+			},
+			// Verify changing some application settings
+			{
+				Config: `
+					resource "gitlab_application_settings" "this" {
+						after_sign_up_text = "Welcome to GitLab!"
+					}
+				`,
+			},
+		},
+	})
+}