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.

未指定文件或测试。首先定位 Splittable DoFn restriction-provider API 及其构造路径；完成标准是 DoFn 实例可以选择充当自身的 restriction provider，而不是要求一个单独绑定的 provider class。

由索引模型根据 Issue 内容生成。