[proposal] Plugin system

[gjalves]

Problem

Integrations or extensions are very hard to mantain if not embedded on mainstream codebase. Sometimes this code does not make sense to be used in the community, or not intended to be shared.

Solution

The idea is to create event hooks in similar fashion of those used on PostgreSQL database, and a configuration entry with a ordered loadable extensions. Those extensions are just ELF libraries with some exported functions to be triggered on specific events.

These hooks will be triggered on the main events of a session, like attach, detach or authentication.

Non-goals

It's not objetive of this proposal to transform the already written codebase into extensions integrated by this hook system

[gjalves]

This is the sketch of the changes to be made:

gjalves@b8002c0