Make IdentitySchema Attributes Available as Resource References

## Feature Request: Make IdentitySchema Attributes Available as Resource References

### Current Behavior

When a resource defines an `id` attribute only in the `IdentitySchema` (via `resource.ResourceWithIdentity` interface), this attribute is not available for reference by other resources in Terraform configurations.

**Example:**

```go
// Resource with IdentitySchema only
func (r *ResourceKubernetes) IdentitySchema(ctx context.Context, req resource.IdentitySchemaRequest, res *resource.IdentitySchemaResponse) {
res.IdentitySchema = identityschema.Schema{
Attributes: map[string]identityschema.Attribute{
"id": identityschema.StringAttribute{RequiredForImport: true},
},
}
}

func (r ResourceR) Schema(_ context.Context, req resource.SchemaRequest, resp *resource.SchemaResponse) {
resp.Schema = schema.Schema{
Attributes: map[string]schema.Attribute{
"name": schema.StringAttribute{Required: true},
// Note: no "id" attribute here
},
}
}
```

**Attempted Usage:**

```hcl

resource "clevercloud_s" "nodes" {
kubernetes_id = clevercloud_r.cluster.id # ❌ Error: Unsupported attribute
name = "worker-nodes"
flavor = "XS"
size = 3
}
```

**Error:**
```
Error: Unsupported attribute

on terraform_plugin_test.tf line 20, in resource "clevercloud_s":
20: kubernetes_id = "${clevercloud_r.cluster.id}"

This object does not have an attribute named "id".
```

### Expected Behavior

Attributes defined in `IdentitySchema` should be automatically available as computed attributes in the resource's main schema, allowing them to be referenced by other resources without requiring manual duplication.

### Current Workaround

Currently, providers must manually duplicate the `id` attribute in both schemas:

```go
func (r ResourceR) Schema(_ context.Context, req resource.SchemaRequest, resp *resource.SchemaResponse) {
resp.Schema = schema.Schema{
Attributes: map[string]schema.Attribute{
"id": schema.StringAttribute{Computed: true, PlanModifiers: []planmodifier.String{stringplanmodifier.UseStateForUnknown()}},
"name": schema.StringAttribute{Required: true},
"kubeconfig": schema.StringAttribute{Computed: true},
},
}
}

func (r *ResourceR) IdentitySchema(ctx context.Context, req resource.IdentitySchemaRequest, res *resource.IdentitySchemaResponse) {
res.IdentitySchema = identityschema.Schema{
Attributes: map[string]identityschema.Attribute{
"id": identityschema.StringAttribute{RequiredForImport: true},
},
}
}
```

This creates unnecessary duplication and potential for inconsistency between the two schemas.