feat: Onboard affinity groups resource and data source (#652)

* onboard affinity_groups resource and data source
- add tests and descriptions
- fix: server doesn't use affinity_group value for payload

* Update descriptions

stackit/internal/services/iaas/affinitygroup/resource_test.go

@@ -0,0 +1,114 @@
+package affinitygroup
+
+import (
+	"context"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/hashicorp/terraform-plugin-framework/types"
+	"github.com/stackitcloud/stackit-sdk-go/core/utils"
+	"github.com/stackitcloud/stackit-sdk-go/services/iaas"
+)
+
+func TestMapFields(t *testing.T) {
+	tests := []struct {
+		description string
+		state       Model
+		input       *iaas.AffinityGroup
+		expected    Model
+		isValid     bool
+	}{
+		{
+			"default_values",
+			Model{
+				ProjectId:       types.StringValue("pid"),
+				AffinityGroupId: types.StringValue("aid"),
+			},
+			&iaas.AffinityGroup{
+				Id: utils.Ptr("aid"),
+			},
+			Model{
+				Id:              types.StringValue("pid,aid"),
+				ProjectId:       types.StringValue("pid"),
+				AffinityGroupId: types.StringValue("aid"),
+				Name:            types.StringNull(),
+				Policy:          types.StringNull(),
+				Members:         types.ListNull(types.StringType),
+			},
+			true,
+		},
+		{
+			"response_nil_fail",
+			Model{},
+			nil,
+			Model{},
+			false,
+		},
+		{
+			"no_affinity_group_id",
+			Model{
+				ProjectId: types.StringValue("pid"),
+			},
+			&iaas.AffinityGroup{},
+			Model{},
+			false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.description, func(t *testing.T) {
+			err := mapFields(context.Background(), tt.input, &tt.state)
+			if !tt.isValid && err == nil {
+				t.Fatalf("Should have failed")
+			}
+			if tt.isValid && err != nil {
+				t.Fatalf("Should not have failed")
+			}
+			if tt.isValid {
+				diff := cmp.Diff(tt.state, tt.expected)
+				if diff != "" {
+					t.Fatalf("Data does not match: %v", diff)
+				}
+			}
+		})
+	}
+}
+
+func TestToCreatePayload(t *testing.T) {
+	tests := []struct {
+		description string
+		input       *Model
+		expected    *iaas.CreateAffinityGroupPayload
+		isValid     bool
+	}{
+		{
+			"default",
+			&Model{
+				ProjectId: types.StringValue("pid"),
+				Name:      types.StringValue("name"),
+				Policy:    types.StringValue("policy"),
+			},
+			&iaas.CreateAffinityGroupPayload{
+				Name:   utils.Ptr("name"),
+				Policy: utils.Ptr("policy"),
+			},
+			true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.description, func(t *testing.T) {
+			output, err := toCreatePayload(tt.input)
+			if !tt.isValid && err == nil {
+				t.Fatalf("Should have failed")
+			}
+			if tt.isValid && err != nil {
+				t.Fatalf("Should not have failed: %v", err)
+			}
+			if tt.isValid {
+				diff := cmp.Diff(output, tt.expected)
+				if diff != "" {
+					t.Fatalf("Data does not match: %s", diff)
+				}
+			}
+		})
+	}
+}