Extending EESSI with different approaches

[ocaisa]

We can create modules to help users to extend EESSI:

• Using EasyBuild (see Easyconfig that can extend EESSI #371)
• Using other build tools (e.g., Spack 😱)
• Manually (update/customise the buildenv easyblock to base itself off of CMakeMake and also install the wrappers)