[PP-1] HTTP API: update /xb-render-component/{component_id} to use Component config entity's default values

Problem/Motivation

🛑 Blocked on ✨ Allow specifying default props values when opting an SDC in for XB Needs work .

SDC's optionally have examples.
⇒ They're not guaranteed to be present.
⇒ A preview is not guaranteed to work. 😱

Steps to reproduce

Currently (visualized using 📌 Connect client & server, with zero changes to client (UI): rough working endpoints that mimic the UI's mocks Needs review ):

Proposed resolution

1. Require every opted in SDC to have a default value specified for every required SDC prop — ✨ Allow specifying default props values when opting an SDC in for XB Needs work is implementing that.
2. Update \Drupal\experience_builder\Controller\SdcController::renderComponent() to use that new default.
3. Delete \Drupal\experience_builder\Controller\SdcController::populatePropValues()

Remaining tasks

See above.

User interface changes

The crash in the GIF will no longer occur.

API changes

None.

Data model changes

None.