Additional modes for handling duplicates in `MutableStep.add_decorator`

[valayDave]

MutableStep currently provides 3 cases for handling duplicates (IGNORE, OVERRIDE, ERROR) when we call MutableStep.add_decorator. These are useful when we are dealing with first class metaflow decorators. But when users customize decorators with @user_step_decroator, users might want to keep all decorators added via mutable_step.add_decorator and even the ones present in the source.

Supporting an additional mode for duplicates would help resolve this issue (maybe ALLOWED ?)