Support beta functionality (#412)

* support beta functionality

* add testing, improve logic and code quality

* improve testing and code quality

* Fix warning message

stackit/internal/features/beta.go

@@ -0,0 +1,48 @@
+package features
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/hashicorp/terraform-plugin-framework/diag"
+	"github.com/stackitcloud/terraform-provider-stackit/stackit/internal/core"
+)
+
+// BetaResourcesEnabled returns whether this provider has BETA functionality enabled.
+//
+// In order of precedence, beta functionality can be managed by:
+//   - Environment Variable `STACKIT_TF_ENABLE_BETA_RESOURCES` - `true` is enabled, `false` is disabled.
+//   - Provider configuration feature flag `enable_beta` - `true` is enabled, `false` is disabled.
+func BetaResourcesEnabled(ctx context.Context, data *core.ProviderData, diags *diag.Diagnostics) bool {
+	value, set := os.LookupEnv("STACKIT_TF_ENABLE_BETA_RESOURCES")
+	if set {
+		if strings.EqualFold(value, "true") {
+			return true
+		}
+		if strings.EqualFold(value, "false") {
+			return false
+		}
+		warnDetails := fmt.Sprintf(`The value of the environment variable that enables BETA functionality must be either "true" or "false", got %q.
+Defaulting to the provider feature flag.`, value)
+		core.LogAndAddWarning(ctx, diags, "Invalid value for STACKIT_TF_ENABLE_BETA_RESOURCES environment variable.", warnDetails)
+	}
+	// ProviderData should always be set, but we check just in case
+	if data == nil {
+		return false
+	}
+	return data.EnableBetaResources
+}
+
+// CheckBetaResourcesEnabled is a helper function to log and add a warning or error if the BETA functionality is not enabled.
+//
+// Should be called in the Configure method of a BETA resource.
+// Then, check for Errors in the diags using the diags.HasError() method.
+func CheckBetaResourcesEnabled(ctx context.Context, data *core.ProviderData, diags *diag.Diagnostics, resourceName string) {
+	if !BetaResourcesEnabled(ctx, data, diags) {
+		core.LogAndAddErrorBeta(ctx, diags, resourceName)
+		return
+	}
+	core.LogAndAddWarningBeta(ctx, diags, resourceName)
+}