New Client Method: Toggle Example as Done/Undone

### Feature description

Given a project_id and an example_id, this new method would be able to update the Done / Undone status of an annotation sample in a Doccano project.

This feature would be tremendously useful to programatically mark annotations as Done when labels are submitted for an example. (rather than requiring annotators to handle this extra workload, which can be significant for large datasets)

c.c: @Hironsan @david-engelmann