Feature request: Enable password to be a callable

[rhoban13]

What would it take to enable a client to pass a callable instead of a string for the password? Based on my read it looks like this is passed into the Transport, which each implementation uses slightly differently upon establishing a connection. So would each transport need to check if the password is a callable, and then invoke it to get the string?

Use case: Related to celery/celery#9454, I'm looking for a clean way to support frequently rotating passwords without the need to restart workers. Our passwords are somewhat short lived identity tokens. I was able to hack it up by creating a custom Transport, but that doesn't work well with the Inspect API.

It feels like this may be a much cleaner way to support changing passwords, but I'm not certain if I'm overlooking some difficulty.