Form Builder Util

## Feature Request

The AdminPage component comes with a `buildSettingComponent` method that converts a config object into a Flarum-specific form field. We could generalize this to an abstract form element builder component, which could help reduce boilerplate and standardize styles.

https://github.com/flarum/core/pull/2996#discussion_r691807155

Start by reading the AdminPage component and its buildSettingComponent method to understand how the current config-to-form-field conversion works. Define the scope of an abstract form element builder and confirm that it reduces the existing boilerplate while preserving standardized styles; the issue names no tests or implementation file.

Written by the indexing model from the issue text.