Move object_sizes to capdl

This target creates a YAML file that is intended to be consumed
by the python-capdl-tool tool and as that tool is used by projects that
don't necessarily use camkes. Moving this to the capdl repo allows
projects to use capdl without having to also use camkes repo
dependencies. It also more clearly indicates that this fullfilling an
interface that the capdl tools define and require and isn't a camkes
specific implementation.

camkes/runner: parse object_sizes from libsel4 for capdl

- add a target object_sizes which generates the object_sizes file
- pass this file to the camkes runner
- call the target in both camkes-top-level.cmake and CMakeLists.txt, as
projects use either of these.

Start reorganising camkes project structure

This adds a CMakeLists.txt file for building camkes libraries to support
projects using camkes without using the provided toplevel CMakeLists file camkes provides.
It is cleaner for a project to import all of the camkes helpers itself
if it wants to build a camkes project. This also allows the generated
camkes-gen.cmake file to be imported into the same CMake scope as the
component declaration calls, making it possible to declare source
files for Camkes components that have their own generation build rules.