Convert plugin configuration to Config Entities

Problem/Motivation

Per suggestion by @jcnventura in #3374845-6: Roadmap for 2.0.0 release →

I would rather move to have config entities, so that you could have multiple (or zero) instances of the plugins with user-defined identifiers.

Taking that a slight step further, this allows each instance to have its own unique configuration as well.

This could also be useful for enabling lifecycle management tools in the future (such as tools to help administrators migrate users from one plugin to another should they decide to decommission a plugin)

May partially supersede 📌 Convert TFA plugin forms to use subforms Active due to touching similar area of the UI.

Steps to reproduce

N/A

Proposed resolution

Convert plugins configuration to use Configuration entities instead of storing configuration in settings.

Remaining tasks

User interface changes

TBD

API changes

TBD

Data model changes

Configuration data will no longer be stored in settings. This will impact TFA's internal storage and may or may not impact plugin storage of data.