Layout Builder's ConfigureSectionForm now has public methods to get the current section and layout

This change allows Layout Builder's /core/modules/layout_builder/src/Form/ConfigureSectionForm.php to expose the current section and its layout via public getters so that it can be altered by contrib modules. In addition, new sections are now initially created with the form rather than being created only on form submission. This allows module developers to alter the initial form based on information in the new session and layout and to then save extra configuration to the layout or thirdPartySettings to the section.

New public methods:

getCurrentLayout()
getCurrentSection()