Builder and getter APIs for components

[Tomboyo]

For all mandatory components,

• The builder API must have a compose$Type($Type) setter
• The getter API must have a $Type as$Type() getter

MyComponent foo = MyModel.newBuilder()
    .composeMyComponenet(instance)
    .buildUnvalidated()
    .asMyComponent();

For all non-mandatory componenets,

• The getter API must instead be optionally typed, as Optional<$Type> as$Type()
• Optionals must be empty if any mandatory property of a component is null on the composed model
• Optionals must be nonempty if every mandatory property of a component is non-null
• Non-mandatory properties have no impact on whether Optionals are empty

Optional<MyComponent> foo = MyModel.newBuilder()
    .composeMyComponenet(instance)
    .buildUnvalidated()
    .asMyComponent();