Add Kafka bindings

[m-wild]

Kafka is a popular message broker.
We should provide

• classes for Kafka bindings,
• documentation on how to use Kafka bindings

https://github.com/asyncapi/bindings/tree/master/kafka

Bindings are currently usable by creating a filter which adds the binding.
e.g.

services.AddAsyncApiSchemaGeneration(options =>
{
    options.ChannelItemFilters.Add(new MyAmqpChannelFilter());
}

class MyAmqpChannelFilter : IChannelItemFilter
{
    public void Apply(ChannelItem channelItem, ChannelItemFilterContext context)
    {
        channelItem.Bindings.Amqp = new AmqpChannelBinding
        {
            Is = AmqpChannelBindingIs.RoutingKey,
            Exchange = new AmqpChannelBindingExchange
            {
                Name = "streetlights",
                Type = "topic",
            }
        };
    }
}

[HUTCHHUTCHHUTCH]

have put in a pr for this issue