Add /healtcheck endpoint

[metobom]

As we are planning to make partner API providers deploy Signed API, we need to have a control mechanism over those deployments. For that, we need a new endpoint named /healtcheck that returns the contents of the Airnode feed heartbeat.

${airnodeAddress} - ${stage} - ${airnodeFeedVersion} - ${currentTimestamp} - ${deploymentTimestamp} - ${airnodeFeedConfigHash} - ${heartbeatSignature}