Handle Structured CloudEvent binary payload convertion

[radhakrishnan10192]

Fixes #1455

Problem

When a structured-mode CloudEvent (Content-Type: application/cloudevents+json, whole envelope in the body) is consumed by a function whose target type is the CNCF
io.cloudevents.CloudEvent, conversion fails.

CloudEventMessageUtils.toCanonical(...) transforms the structured message into a binary-shaped message (data extracted into the payload, attributes moved to ce-* headers), but
the resulting message is left in an inconsistent state:

1. Content-type isn't reset — the message still says application/cloudevents+json even though the payload is no longer the serialized envelope.
2. data is left as a deserialized Map, not serialized bytes.

Downstream, io.cloudevents.spring.messaging.CloudEventMessageConverter re-decides structured-vs-binary purely from contentType. Because the stale application/cloudevents+json
remains, it selects the structured reader and tries to re-parse the already-extracted payload as the envelope:

io.cloudevents.rw.CloudEventRWException: Could not parse. Unknown encoding. Invalid content type or spec version                                                               
       at io.cloudevents.core.message.impl.MessageUtils.parseStructuredOrBinaryMessage(MessageUtils.java:69)                                                                    
       at io.cloudevents.spring.messaging.CloudEventMessageConverter.fromMessage(CloudEventMessageConverter.java:45)                                                            
       at ...SmartCompositeMessageConverter.fromMessage(...)                                                                                                                    
                                                                                                              
and 

java.lang.IllegalArgumentException: argument "src" is null
   at io.cloudevents.jackson.JsonFormat.deserialize(JsonFormat.java:158)
   at io.cloudevents.spring.messaging.CloudEventMessageConverter.fromMessage(CloudEventMessageConverter.java:45)
   at org.springframework.cloud.function.context.config.SmartCompositeMessageConverter.fromMessage(...)
   at ...SimpleFunctionRegistry$FunctionInvocationWrapper.convertInputMessageIfNecessary(...)
                                                                 

This only affects the io.cloudevents.CloudEvent target type. A plain POJO target works because Spring's own converters can bind a Map payload to a POJO, which is why the existing tests didn't catch it.