Introduce configurable plugins

[EclipseGc]

Configurable plugins should be part of the basic offering of Plugins. In Drupal 8 we did this via arrays for configuration, but this means that the plugin was in charge of things like validating configuration, serializing itself for storage, etc, etc.

We should write this such that a given plugin expects a given configuration object (which extends a common interface). This configuration object should be capable of validating its values and serializing its values.

[mikeryan776]

First question is, are you still pursuing the plugins library? Assuming you are, have you looked at existing libraries to use here? I have a similar need and identified https://packagist.org/packages/symfony/options-resolver and https://packagist.org/packages/hassankhan/config as possibilities. I haven't looked that closely yet, but leaning in the Symfony direction here (not least because it does include validation).