Eventbrite: Add Spacing Option for Button style

Just like the [Subscription block that is being updated](https://github.com/Automattic/jetpack/pull/15107), it'd be nice if we could change the spacing inside of the button so that the customers can make the button wider.

Review the linked Subscription block pull request (#15107), then locate the Eventbrite button style entry point in the Jetpack repository. Confirm how button spacing is currently handled; done means users can adjust the spacing inside the button so it becomes wider, with relevant tests updated if present.

Written by the indexing model from the issue text.