Add default configuration values to BlockSource

Problem/Motivation

While working on #3571451: Check config schema when attaching and rendering a block, we noticed than block sources are not initialized with their default values.

In UI Patterns 2, we don't care much about required block settings because it was managed by the Form API:

1. if a setting is required, we add the #required render property
2. BlockForm can's be submitted without filling a required value
3. So it is not possible to render a block without required props filled

But in Display Builder, we can render component without subimiting BlockForm, so with a missing required value:

• on preview panel
• on builder panel

Proposed resolution

That's why we need to add those default values.