[Feature Request] Support activity delete

[ferhatelmas]

Users should be able to delete their posts.

A similar request: https://stackoverflow.com/questions/61972374/getstream-com-question-how-can-a-user-that-posted-something-delete-edit-his-pos

This might be incorporated to changes within #124.

/cc @vishalnarkhede

[GuyAvraham]

Thank you @ferhatelmas