Meta Description Generator: Settings Page

### Description

>[!TIP]
>[Meta Description Generator feature reference document for Alley devs](https://l.alley.dev/0e3715e20a)

Set up a settings page that allows users to choose which AI connector to use when generating meta descriptions.

Use the Settings API to build the admin fields. We don't want to rely on other plugins for building admin fields.

Build as a feature, not a Singleton.

We should add a filter to allow developers to manage which AI service provider is selected via code. If a developer has utilized the filter to set the selected AI service provider via code, the settings should still be registered so that WordPress knows about them, but should not be editable on the settings screen (the settings screen should not appear in the admin OR should be read-only).

### Use Case

When an editor or developer wants to enable AI features in WP SEO, they are able to easily select an AI connector that was already configured in WordPress. They should also be able to select different providers for different features, if desired, or select no provider if support should not be given for that feature.

### Notes

Should be done after #162