tfsdk: Add support for single nested mode blocks (#477)

Reference: #400

This support is required for provider developers to migrate from terraform-plugin-sdk to terraform-plugin-framework without affecting practitioner configurations or requiring state upgrades.

For example:

```terraform
resource "examplecloud_thing" "example" {
  timeouts {
    create = "5m"
  }
}
```

.changelog/477.txt

@@ -0,0 +1,7 @@
+```release-note:note
+tfsdk: Schema definitions may now introduce single nested mode blocks, however this support is only intended for migrating terraform-plugin-sdk timeouts blocks. New implementations should prefer single nested attributes instead.
+```
+
+```release-note:enhancement
+tfsdk: Added single nested mode block support
+```

internal/fwschema/block_nested_mode.go

@@ -3,10 +3,11 @@ package fwschema
 // BlockNestingMode is an enum type of the ways attributes and blocks can be
 // nested in a block. They can be a list or a set.
 //
-// While the protocol and theoretically Terraform itself support map, single,
-// and group nesting modes, this framework intentionally only supports list
-// and set blocks as those other modes were not typically implemented or
-// tested since the older Terraform Plugin SDK did not support them.
+// While the protocol and theoretically Terraform itself support map and group
+// nesting modes, this framework intentionally only supports list, set, and
+// single blocks as those other modes were not typically implemented or
+// tested with Terraform since the older Terraform Plugin SDK did not support
+// them.
 type BlockNestingMode uint8
 
 const (
@@ -23,4 +24,13 @@ const (
 	// with multiple, unique instances of those attributes nested inside a
 	// set under another attribute.
 	BlockNestingModeSet BlockNestingMode = 2
+
+	// BlockNestingModeSingle is for attributes that represent a single object.
+	// The object cannot be repeated in the practitioner configuration.
+	//
+	// While the framework implements support for this block nesting mode, it
+	// is not thoroughly tested in production Terraform environments beyond the
+	// resource timeouts block from the older Terraform Plugin SDK. Use single
+	// nested attributes for new implementations instead.
+	BlockNestingModeSingle BlockNestingMode = 3
 )