Add a setting to enable Orders Autosuggest

[JakePT]

Description of the Change

Adds a setting to enable Orders Autosuggest. The setting is disabled for non-EPIO users, with a filter to enable it.

Enabling Orders Autosuggest will also enable indexing of Orders, regardless of whether the Protected Content feature is enabled. Therefore a reindex is required when enabling the feature. This includes a change to allow requiring a reindex if a particular feature setting is changed.

Closes #3277

How to test the Change

• Disable Protected Content feature.
• Enable Orders Autosuggest from the WooCommerce feature. You should be warned that a reindex will be required.
• Visit WooCommerce > Orders and start searching for orders. Suggestions should be offered in a listbox below the search form.
• If not using EP.io the option to enable the feature should be disabled, with an explanatory message.

Changelog Entry

See #3175

Credits

@JakePT @felipeelia

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[anjulahettige]

The option will be grayed out to non-EPio users with the following message:
Due to the sensitive nature of orders, this autosuggest feature is available only to ElasticPress.io customers. (Learn More)

[JakePT]

@anjulahettige @felipeelia @brandwaffle I've pushed a change with the message, but I still need a URL for the Learn more link.