Add the possibility to process or transform the message

I need to be able to transform some messages. For example obfuscate some information. I want to be able to do that at the encoder level and not before on several components.

Now a days, the encode method add the message using
`event.getFormattedMessage()`

It could be very useful if we can add something similar to the following at the ECSEncoder class:

/**
* Subclasses can override this to format the message (for example to obfuscate some fields.)
* @param event The event to be logged
* @return The message processed. By default, this method return the message without any modification
*/
protected String processMessage (ILoggingEvent event){
return event.getFormattedMessage();
}

And at the encode method

`EcsJsonSerializer.serializeFormattedMessage(builder, processMessage(event));`

What do you think?

The method name could be process, transform, or just getMessage...