Generalize validation options

Currently there are two types of validations supported: ttbar (validation_file) and Z' (add_validation_file). To add a third type, i.e. combined ttbar and Z', is not currently possible without loosing either ttbar or Z' validations separately. To solve this issue, the proposed solution would involve a generalization of validations options. They would follow a format like the following inside the training config (i.e. umami/examples/DL1r-PFlow-Training-config.yaml):

validation_files:
    ttbar: <path_to_ttbar_validation_file>
    zprime: <path_to_zprime_validation_file>
    hybrid: <path_to_hybrid_validation_file>