[6.x] Add option for addons to opt-out of config layouts for settings

[duncanmcclean]

Addon Settings publish forms currently use the "config" (50 / 50) layout for displaying fields.

While we aim for all configuration & settings pages to use the config layout, this PR adds an option for addons to opt-out and use the "content" layout instead (the layout used on entries, terms, etc) if it better suits their use case:

// src/ServiceProvider.php

public function bootAddon()
{
    $this->useContentLayoutForSettings();
}

Closes #12810

[jasonvarga]

This might be good to have, but the issue described in #12810 feels like a bug. The config field layout should only be applied to fields at the top level.

[duncanmcclean]

This might be good to have, but the issue described in #12810 feels like a bug. The config field layout should only be applied to fields at the top level.

Makes sense! I've opened #12882 to address this.