Allow configurable hostname lookup

[jtomson]

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We are using this interface to perform IPv4-only lookups. It may also help those looking for other hostname resolution behavior like https://github.com/eclipse/paho.mqtt.java/issues/475

If there's interest in merging this I am happy to add javadocs as well as clientv5 modifications

[jtomson]

@rdasgupt is there any appetite for this change? if so I'd be happy to resolve the conflicts thanks