Support Spring Data projections as `@SchemaMapping` method return values

[rstoyanchev]

QueruydslDataFetcher and QueryByExampleDataFetcher already support use of projection interfaces to customize return values. The same is useful in controller methods too to allow loose coupling between schema types and domain entities, see for example discussion in #636.

An @ProjectAs annotation on the controller method could be used to specify the projection interface type to apply to the return value.