Add the ability to add another user as watcher on an issue

[mur4s4m3]

Is your feature request related to a problem? Please describe.
This in not implemented yet, although method signature looks promising

Describe the solution you'd like
be able to provide a user to the addWatcher method to add another user instead of just self-assign as it's not usable for automation cases where we have a technical user doing actions.

Describe alternatives you've considered
n/a

Additional context
PR #291