Added iot core config-in node

[nated0g]

Hi @kolban-google.

As discussed in #99, I was looking for a way to receive config objects from IoT Core. I added a node for this (mostly it just involved using parts of other nodes already existing in this repository.)

It is fully working for me, the only issue I see is that in future the telemetry-send node and this node should share a connection to the Google MQTT bridge. Presently if there are telemetry-send and config-in nodes on the same flow configured for the same device, both nodes periodically reconnect, causing the config-in node to repeatedly receive the newest config when it re-subscribes after reconnection, about every 20 seconds or so.

I propose a shared configuration node for all IoT Core related nodes to allow for a single persistent connection to the bridge.

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[nated0g]

@googlebot I signed it!

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[nated0g]

Should have made changes on dev branch on my fork. Resubmitting