Add parsed configuration properties

[MortalFlesh]

Hello,

I've parsed all properties from kafka configuration here https://github.com/edenhill/librdkafka/blob/master/CONFIGURATION.md into PHP classes with constants and proper description.

So for example

Property	C/P	Range	Default	Description
client.id	*		rdkafka	Client identifier. Type: string
message.max.bytes	*	1000 .. 1000000000	1000000	Maximum Kafka protocol request message size. Type: integer

Will be

class GlobalConfig
{
    /**
     * Client identifier.
     *
     * Type: string
     * Default: rdkafka
     */
    public const CLIENT_ID = 'client.id';

    /**
     * Maximum Kafka protocol request message size.
     *
     * Type: integer
     * Range: 1000 .. 1000000000
     * Default: 1000000
     */
    public const MESSAGE_MAX_BYTES = 'message.max.bytes';
}

So I have classes

PHP class	Description	Consumer/Producer	Note
GlobalConfig.php	Global configuration properties	Both
GlobalConsumer.php	Global configuration properties	Consumer	extends GlobalConfig
GlobalProducer.php	Global configuration properties	Producer	extends GlobalConfig
Topic.php	Topic configuration properties	Both
TopicConsumer.php	Topic configuration properties	Consumer	extends Topic
TopicProducer.php	Topic configuration properties	Producer	extends Topic

I created an issue php-enqueue/enqueue-dev#464 in php-enqueue repo, but they pointed me at php-rdkafka extension, but I think, this is better place to put those constants.

I'm not sure about namespace and dir structure.
What do you think?

[aand18]

Wow! It would be nice if this got merged!