Plugin system for execution targets

[vstirbu]

Currently the cpu and the gpu execution targets are hardcoded into the library, even if they are plugging developed with plugs.

The development of new targets or updates to existing ones could be developed independently if the plugins can be separate packages. To support this functionality we need to be able to:

• load targets dynamically based on the metadata included in the package metadata exposed in pyproject.toml
• validate the targets in the CLI based on the installed targets
• formalize the plugin interface