Deprecate provide* methods of UiItemsProvider

[GerardasB]

Describe the Solution you'd like

provide* methods of UiItemsProvider should be replaced by corresponding get* variants.

Tasks

Beta Give feedback

1. Deprecate provide* methods of UiItemsProvider
2. Bump get* methods of UiItemsProvider to @public
3. Bump layouts properties and types to @public i.e. CommonToolbarItem["layouts"], Widget["layouts"]
4. Update WidgetLayouts and ToolbarItemLayouts types to allow for custom layouts: { [layoutName: string]: any }

[ignas-k]

Decided not to implement the custom layout because we do not have the functionality to fully support it and we can easily add it later.