send/change reactive objects into running shiny app from side thread to `module2app()

[maxheld83]

specifically into the reactive inputs to modules, which aren't changed by anything/anyone else in testing inside module2app().

Note to self: I thought about adding UI to change these inputs at some point, but that would quickly get out of hand, because these inputs can be arbitrary and complex.

However, having a reactive domain (or whatever?) into which you can send this from the console would be very helpful for testing.

[maxheld83]

maybe a plumber endpoint or something which listens for inputs?