integrate API Attribute backfiller & Unique String Aggregator to Core

[donrestarone]

Is your feature request related to a problem? Please describe.

While developing app functionality for nikean.org we had to write a 2 custom plugins. One to backfill API Namespace attributes (used when product requirements changed) and another to build lists of unique strings. These plugins were written quickly and are well past their use-by date.

Describe the solution you'd like

Lets move these to Violet Core, put the functionality behind a User Interface and have it maintained as part of the monolith codebase.

These plugins are installed and tested in the test suite, see the following releases that introduced the plugins:

1. Unique String aggregator: https://github.com/restarone/violet_rails/releases/tag/0.9.49
2. Create External API Connection that backfills attributes on API Namespace + API Resources https://github.com/restarone/violet_rails/releases/tag/0.9.25

[donrestarone]

This UI does not change

Plugins tab

requirements:

1. make sure that it can hold any number of plugins (as a list)
2. be able to organize a potentially large number of plugins in an intuitive way

attribute backfiller requirements

let the user define the attribute and the default value: