MQTT Binding.

[JemDay]

For comment at present - DO NOT MERGE.

Contains MQTT V3 & V5 binding implementations for the Paho and HiveMQ client libraries.

Closes #335

[JemDay]

@Alfusainey

[Alfusainey]

@Alfusainey

@JemDay LGTM. This is a neat contribution, thanks!

[JemDay]

Thanks @Alfusainey - I do need to clean up the documentation side of it but if there's anything else that appears glaringly wrong pls let me know.

@pierDipi - if you could start taking a peek at this I'd appreciate it.

[pierDipi]

what's rationale for using a public static method vs referencing the field directly and make it public?

[pierDipi]

Should we throw an exception in this case mostly for "internal safety"?

[JemDay]

I think I need to do some general clean-up ..

As an aside do we know why the definition of a 'MessageWriter' interface is so complicated, I'm not sure why it needs to extend other writers and factories.

[pierDipi]

On multiple files the copyright header is missing:

sdk-java/api/src/main/java/io/cloudevents/lang/Nullable.java

Lines 1 to 16 in 4c81f3e

	/*
	* Copyright 2018-Present The CloudEvents Authors
	* <p>
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	* <p>
	* http://www.apache.org/licenses/LICENSE-2.0
	* <p>
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*
	*/

[pierDipi]

btw, I'm still going through it

[JemDay]

thx ... I'll try and take a look at this next week now that the proto changes are in ...