allow any attribute in secondary_info on entity

[Mariusthvdb]

would be very nice if we could do:

              - entity: sensor.den_haag_hs_roosendaal
                secondary_info: arrival_time_planned

without having to resort to custom cards

              - type: custom:template-entity-row
                entity: sensor.den_haag_hs_roosendaal
                secondary: >
                  Aankomst: {{state_attr(config.entity,'arrival_time_planned')}}

or double listing of entities

              - entity: sensor.den_haag_hs_roosendaal
                #secondary_info: arrival_time_planned
              - type: attribute
                entity: sensor.den_haag_hs_roosendaal
                attribute: arrival_time_planned