cmake: add ZephyrAppConfiguration package

Fixes #56200
Add the ZephyrAppConfiguration package and update the corresponding
documentation. This adds flexibility to the CMake build configuration
by providing a workspace configuration package and an application
package, which only applies to the current application. The workspace
package stays the same as before, but the application package is
new and lives, per default, inside the application folder.

Signed-off-by: Nico Lüthi <nylnx@outlook.com>