support IotGateway use with IotProvider and PublishSubscribeService

Support for IotGateway and IotpGateway was added in EDGENT-377.
See the IotpGateway javadoc regarding limitations of use with IotProvider.

Add support for supplying an IotGateway to initialize IotProvider and pub-sub support to use/expose proxies for an IotGateway (the ability to publish events for a gateway's connected devices and to subscribe to commands for them). Address the "not yet implemented" PubSubIotDevice.getDeviceType() and getDeviceId()

Implementation thoughts:

• perhaps create "PubSubIotGateway implements IotGateway" (may wrap or extend PubSubIotDevice).
• the IotDevicePubSub application needs "IotGateway addIotGateway()", which would return a PubSubIotGateway.