Allow Splittable DoFn itself to be used as the restriction provider.

Binding the provider to the DoFn class itself at construction time can be limiting, we should add an option to use the DoFn instance itself as the restriction provider.

Imported from Jira [BEAM-11354](https://issues.apache.org/jira/browse/BEAM-11354). Original Jira may contain additional context.
Reported by: robertwb.

未指定檔案或測試。首先定位 Splittable DoFn restriction-provider API 及其建構路徑；完成標準是 DoFn 執行個體可以選擇充當自身的 restriction provider，而不是要求一個個別繫結的 provider class。

由索引模型根據 Issue 內容生成。