[DMN Designer] Decision Service Output Data-type should be based on Output Decisions

The Output Data-type of a Decision Service can currently be set explicitly by the User (and indeed must be set by the User). However the Output Data-type of a Decision Service can be automatically set (and possibly created) based on the Output Data-type of the Output Decision(s) within the Decision Service.

For example, if the Decision Service contains a single Output Decision the Output Data-type of the Decision Service is the same as that of the single Output Decision. If the Decision Service has multiple Output Decisions then the Decision Service Output Data-type should be a combination of those of the different Output Decisions. A new Data-type may need to be automatically generated. For example a DS has 2 OD's with Data-types A and B. The DS Output Data-type should be a single Data-type being the combination of A and B.