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.

No files or tests are named. Start by locating the Splittable DoFn restriction-provider API and its construction path; done means a DoFn instance can optionally serve as its own restriction provider instead of requiring a separately bound provider class.

Scritto dal modello di indicizzazione a partire dal testo della issue.