Issue #3612604 by rymcveigh: Cast third-party settings checkboxes to real...

Issue #3612604 by rymcveigh: Cast third-party settings checkboxes to real booleans and complete config schema

The forward_navigation and prevent_next_validation checkboxes on the webform third-party settings form had no #element_validate callback, so Form API's raw '1'/'0' submission values were saved directly instead of real booleans. This was compounded by the config schema only declaring a type for forward_navigation, so schema validation had no way to catch the bad values being stored for prevent_next_validation.

• Add #element_validate casting to both checkboxes.
• Complete the config schema with entries for prevent_next_validation, additional_error_message, and the webform_navigation handler's debug setting.
• Add an update hook to normalize any non-boolean values already stored on existing webforms.

Closes #3612604