AFT-702: Add heterogeneous input support for GN2

Idea:
Create an abstract class for a constituent loader with the getData function, which returns a vector of values. Each loader (e.g., TracksLoader, IParticlesLoader, ...) is inherited from it. Then, we have a vector of pointers and sequentially apply all of them and feed output to GNN. Relates to AFT-702

TO-DOs:

• Add abstract class for constituent loading
• Add tracksLoader class
• Add tracksLoader configuration
• Add IParticleLoader class
• Clean code

Tagging @dguest @svanstro @ioleksiy