Dynamic loading for plugins

[cedricchevalier19]

The goal is for Alien to load third party library plugins dynamically.
As a consequence, Alien libraries are not any more known at compile time and the template way to dispatch across plugin is not possible.

Base features:

• Define a minimal interface for plugins (functions and symbols to explicitly resolve) #141
• Implement loader (for Linux / Unix) #142
• Refactor Alien dispatch to be compatible with dynamic plugin #143

Advanced features:

• Distinguish between plugin flavors: allow several back-ends for a library (Serial, OMP, Cuda, HIP, ...) #144
• "Containerize" plugins so that several implementations of the same plugin can be use at the same time, without interference #145
• Dynamic loading for Windows