public ModelAssembler addTrait(ShapeId target, Trait trait) {
        this.pendingTraits.add(Pair.of(target, trait));
        return this;
    }
Explicitly adds a trait to a shape in the assembled model.

@param target Shape to add the trait to.
@param trait Trait to add.
@return Returns the assembler.
smithy-model/src/main/java/software/amazon/smithy/model/loader/ModelAssembler.java