Make it possible to hook/subscribe to prompts being built

Problem/Motivation

Currently there is just a prompt file and that is it. This makes it hard to adapt to all the special cases an API might have or the flexibility they have to offer.

One example is the field agent and the storage settings for the module Custom Field. Since it offers dynamic ways of storing fields, it's not a fixed field structure and most likely requires it own few shot learning to get right. It might even need to add a dynamic list of options available that is just not showing up on the initial config or form.

Steps to reproduce

Proposed resolution

* Check if eventsubscribers or hooks is the best solution.
* Make it possible to listen for just one specific file/agent combination.
* Make it possible to change the instruction text.
* Make it possible to add or remove one shot prompting
* Make it possible to add context

Remaining tasks

User interface changes

API changes

Data model changes