Inherit fields from parent classes

[dariusmb]

Description

Inherit the fields from all parent classes.

e.g:

class BaseSchema extends Schema<BasePayload> {
  @StringField()
  parentField: string;
}

class ChildSchema extends Schema<ChildPayload> {
  @IntegerField()
  extraField: string;
}

When the ChildSchema is validated, it should also validate the parentField