Add SafeVarargs annotations

[hpoettker]

This PR adds @SafeVarargs to methods in Chunk and CompositeItemWriter that take a generic varargs parameter but don't perform unsafe operations on them. This prevents false positive warnings in the users' code when they use these methods.

In the constructor of Chunk with the generic varargs parameter, I've replaced Arrays.stream(items).toList() with Arrays.asList(items). Both work fine regarding varargs safety in practice but the latter is explicitly annotated with @SafeVarargs itself, while Arrays.stream does not explicitly say that it works fine with an array of type Object[] in every case. Using Array.asList also saves one copy of the array, which is not needed as the array will be copied later anyway.