A button for subscribing to an in-app subscription.
This button is used by standard subscription store control styles. If you implement a custom
subscription store control style, you can use this button as part of your style by providing a
`SubscriptionStoreControlStyle.Configuration.Option`` value.
On iOS, macOS, watchOS, and visionOS you can configure the label of the button by modifying it with
`subscriptionStoreButtonLabel(_:)`. Some button label configurations cause the button
to have a caption, for example: `.displayName.singleLine`.
You can configure the button's style by modifying it with `buttonStyle(_:)`.
You don't use this directly except when implementing a
`SubscriptionStoreControlStyle`.