Add a 'depends on' field which should hold all the different systems the given host directly depends on

[P-T-I]

Has a relation with #485
It would be nice to document direct dependencies per inventory host; probably the customization specs would be the ideal place to do so.

• The field should fetch dependencies from the inventory hosts already present in providentia
• The field should accept 1 or multiple unique entries (1 system could have direct dependencies to several other systems)
• The field should only have CRUD capabilities for specific users / roles.