Define schema for storage flag in Terraform provider

This commit extends the application schema to support the storage flag in the Terraform provider for Juju.
The schema now includes varios attributes for configuring the storage constraints of a Juju application.

internal/provider/resource_application.go

@@ -6,6 +6,7 @@ package provider
 import (
 	"context"
 	"fmt"
+	"github.com/hashicorp/terraform-plugin-framework/resource/schema/objectplanmodifier"
 	"strings"
 
 	"github.com/hashicorp/terraform-plugin-framework-validators/listvalidator"
@@ -164,6 +165,34 @@ func (r *applicationResource) Schema(_ context.Context, _ resource.SchemaRequest
 					stringplanmodifier.UseStateForUnknown(),
 				},
 			},
+			"storage": schema.SetNestedAttribute{
+				Description: "Configure storage constraints for the juju application.",
+				Optional:    true,
+				NestedObject: schema.NestedAttributeObject{
+					Attributes: map[string]schema.Attribute{
+						"label": schema.StringAttribute{
+							Description: "The specific storage option defined in the charm.",
+							Required:    true,
+						},
+						"size": schema.StringAttribute{
+							Description: "The size of each volume. E.g. 100G",
+							Required:    true,
+						},
+						"pool": schema.StringAttribute{
+							Description: "Name the storage pool to use. E.g. ebs on aws.",
+							Optional:    true,
+						},
+						"count": schema.Int64Attribute{
+							Description: "The number of volumes.",
+							Optional:    true,
+						},
+					},
+					PlanModifiers: []planmodifier.Object{
+						objectplanmodifier.RequiresReplaceIfConfigured(),
+						objectplanmodifier.UseStateForUnknown(),
+					},
+				},
+			},
 			"trust": schema.BoolAttribute{
 				Description: "Set the trust for the application.",
 				Optional:    true,