Allow optional target type to be specified for JavaTemplate

[knutwannheden]

When creating a JavaTemplate it should be possible to optionally specify a target type (as defined in the JLS). This is for example necessary when the template expression is only a method reference or a lambda and would unlock further Refaster use cases as well as declaring some more templates as context-free (like this example).

This idea would be for the JavaTemplate.Builder to declare a new targetType() method. Whether the parameter's type should be a plain String, JavaType, or even have both overloads would have to be explored.

Note that the target type would have to be included in the cache key for the context-free template cache.

An example would be a template like () -> 3 which would not be usable as a context-free template unless a matching target type like Supplier<Integer> is provided. This target type would then allow the code generator to generate a variable declaration for the template, like Supplier<Integer> o = () -> 3;.