Remove UnknownEncodingMessageReader and replace with exceptions

[slinkydeveloper]

• Remove UnknownEncodingMessageReader and, in MessageUtils#parseStructuredOrBinaryMessage, replace the unknownMessageFactory with a CloudEventRWException
• Removed Encoding#UNKNOWN variant since it won't be useful anymore

[smadasu]

@slinkydeveloper TBH, I don't even see the need for this class at all. There is no functionality in the class except throwing exceptions for any method call. This class should be removed and CloudEventRWException should be thrown at the places where this is instantiated.

[slinkydeveloper]

Makes sense, I made a list of things to do, do you wish to contribute it @smadasu?

[smadasu]

Sure @slinkydeveloper . I will create a PR for this.