Subscriptions Block: Add Outline Button Style

Core Button block offers two styles: Fill and Outline. I would like to propose that the button in the Subscriptions block also supports them. While creating the "outline" style buttons is possible with the available color settings, it's not a flexible solution.

Here's a real-life example. Consider this design:

While it's possible to create this pattern in the editor, if the customer changes the background of the parent group block, they will have to go in and change the background of the button as well. It's not a huge deal but would improve the editing experience.

Subscriptions Block color settings:

Button block styles for reference:

Start by locating the Subscriptions block and comparing its button controls with the Core Button block styles. Confirm the work is complete when the Subscriptions block exposes Fill and Outline styles and the editor preview supports the described background-change workflow.

Written by the indexing model from the issue text.