Support primitive function interfaces

[br05s]

When implementing some functions for Spring Cloud Stream, I noticed that it only supports functions that extend from Consumer, Supplier, and Function. Java 8 functions also has support for primitives with the use of interfaces such as IntConsumer, IntSupplier, IntFunction, and ToIntFunction. There are also long and double versions of each.

org.springframework.cloud.function.context.catalog.FunctionTypeUtils has a check in assertSupportedTypes that doesn't check for these other types.

I'd like to see these other interfaces supported so primitive types can be used without the need for boxing/unboxing. I've considered using boxed types, but do not like that approach due to performance impacts.

[olegz]

The level of complexity it would introduce to the current code base would probably be a bit disproportionate to the end benefit. However I did not have a proper chance to investigate it nor has it ever been requested.
In other words it would definitely be of lowest priority. That said if you want to take a shot at it, i would gladly review the PR, so let me know.