Adding support for Reactive Streams `Publisher` return types. Support
is provided through the `ReactiveInvocationHandler` and follows a similar
pattern used by `feign-hystrix`. Each method invocation is wrapped in a
`Callable`, which is then wrapped into the appropriate Reactive Streams
`Publisher`, as defined in the builder and the return type of the method.
This approach is not "reactive all the way down". The requests are still
executed via a regular `Client` and are synchronous. However, it is possible
to still take advantage of the backpressure, scheduling, and functional support
provided by the library implementations.
Limitations: Streams are not supported and Iterable responses are not treated
reactively. Iterables must be explicitly cast into a reactive type.
Reworked Builders and removed the need for the enumerator
Marvin Froeder
Kevin Davis