Feature: #61903 - PageTS dataprovider for backend layouts

See Issue #61903

Description

Over the last year, several extensions appeared on TER that implemented the very same basic feature: Deploying backend layouts without database records by providing them via PageTS.

Implement a generic PageTS provider for backend layouts to unify those approaches and to make backend layouts reusable across installations.

Impact

It is now possible to define backend layouts via PageTSConfig on every page.

Example

mod {
        web_layout {
                BackendLayouts {
                        exampleKey {
                                title = Example
                                config {
                                        backend_layout {
                                                colCount = 1
                                                rowCount = 2
                                                rows {
                                                        1 {
                                                                columns {
                                                                        1 {
                                                                                name = LLL:EXT:frontend/Resources/Private/Language/locallang_ttc.xlf:colPos.I.3
                                                                                colPos = 3
                                                                                colspan = 1
                                                                        }
                                                                }
                                                        }
                                                        2 {
                                                                columns {
                                                                        1 {
                                                                                name = Main
                                                                                colPos = 0
                                                                                colspan = 1
                                                                        }
                                                                }
                                                        }
                                                }
                                        }
                                }
                                icon = EXT:example_extension/Resources/Public/Images/BackendLayouts/default.gif
                        }
                }
        }
}