Fix plugin dependency management in cmake

[James-Mart]

Since changing from custom command/target to ExternalProject_Add for adding rust components (plugins) to the build system, I broke the management of the build order when plugins depend on each other:

https://github.com/gofractally/psibase/pull/656/files?diff=split&w=0

One idea to fix it is to add some top-level cargo project that adds all plugins as members of the same workspace, then in CMake we just add that one rust crate as the external project that manages building all subprojects.