Factor out a new strategy for wrapper type detection

Using this strategy libraries could be developed for supporting
Flux-like libraries (e.g. kstreams) that are not actually
reactive streams implementations.

spring-cloud-function-context/src/main/resources/META-INF/spring.factories

@@ -1,2 +1,5 @@
 org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
-org.springframework.cloud.function.context.config.ContextFunctionCatalogAutoConfiguration
+org.springframework.cloud.function.context.config.ContextFunctionCatalogAutoConfiguration
+
+org.springframework.cloud.function.context.WrapperDetector=\
+org.springframework.cloud.function.context.config.FluxWrapperDetector