ETH-475: heartbeat for nodes in BrokerPool

[jtakalai]

No description provided.

[linear]

ETH-474 BrokerPool additions for miner subsystems

Parent ticket for Tatum smart contract features needed/used by miner plugin in Broker.

ETH-475 BrokerPool heartbeat method

The nodes declared via ETH-472 should be able to announce connectivity-related metadata through a function on the smart contract. The information is intended for off-chain use and doesn't need to be stored in the contract. The contract is therefore (ab)used as a sort of shared communication channel to facilitate, among other things, discovery of entry points to the Streamr Network.

• Add function heartbeat(jsonData) which is callable by the set of nodes declared via ETH-472 or the owner of the BrokerPool (the 'broker')
• The jsonData argument is similar in spirit to the metadata stored in StreamRegistry and ProjectRegistry - a freeform blob intended to be interpreted as a string off-chain. I didn't specify the solidity type here as I'm not sure what's appropriate (a string type or just a byte array).
• The only thing the heartbeat method should do is fire a Heartbeat(nodeAddress, jsonData) event.

[samt1803]

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