Support for conditional requirement of variables

[MB175]

type Specification struct {
	FEATURETOGGLE   bool `required:"true"`
	FEATURESETTING  string `conditionalRequired:"{FEATURETOGGLE == true}"`

}

Feature description:

• Be able to make field a requirement based on the fact if a given condition is met or not.
  For example, if the featuretoogle is turned on make all fields with the conditionalRequired Tag required if their condition is met.

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Feel free to close this right away if this feature is somehow present already, in case I overlooked it.