Arbitrary module prompting of agents

[sjrct]

There needs to be an easy way for modules to send a reply to an agent and await a response. The initial use case for this is to allow a configuration agent to prompt an admin for doing things like adding a module to the existing halibot instance (the prompts from halibot add).

Note that sync_send_to cannot be used as agents do not reply to messages.

I am considering implementing a prompt function on HalModule or a subclass thereof that awaits so future message to be send back to the module.