Changeable invokers for Proxy client

[alex-shpak]

Hi,

This PR will add ResourceMethodInvoker for proxy client factory.
This will allow to substitute client invocation for example to support RxJava proxy client like here
Example:

class RxResourceMethodInvoker implements ResourceMethodInvoker<RxInvocationBuilder> {

    @Override
    public <T> T method(RxInvocationBuilder builder, String name, GenericType<T> responseType) {
        return builder.rx().method...;
    }

    @Override
    public <T> T method(RxInvocationBuilder builder, String name, Entity<?> entity, GenericType<T> responseType) {
        return builder.rx().method...;
    }
}

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