Add a `wait-for-completion` option.

[VincentLanglet]

Hi @benc-uk,

There is a fork of your action (aurelien-baudet/workflow-dispatch) which is providing a wait-for-completion action: https://github.com/aurelien-baudet/workflow-dispatch#wait-for-completion

Looking at github history, it doesn't require a lot of commit https://github.com/aurelien-baudet/workflow-dispatch/commits/master/

But this fork is not maintained anymore (cf https://github.com/aurelien-baudet/workflow-dispatch/pull/16#issuecomment-2103949756) Would you be interested adding such feature here ?