Add Functional Health Checks

[rezarajan]

The current implementation of health checks just returns a success message, which is not that useful in a production setting. There is a TODO in the code

tx-submit-api/internal/api/api.go

Lines 187 to 190 in 67ea49e

	func handleHealthcheck(c *gin.Context) {
	// TODO: add some actual health checking here
	c.JSON(200, gin.H{"failed": false})
	}

Some initial ideas

• Perform a check on the node socket (e.g., if a node fails/restarts during runtime and the socket disappears)
• Perform some dummy checks on the endpoints /api/submit/tx and /api/hastx/:tx_hash for liveness (not necessary to care about responses)

[wolf31o2]

I'm not sure that there is a functional test that makes sense for liveness. Testing the socket or the endpoints is essentially testing the underlying cardano-node, which is best monitored directly. Also, this can use Ouroboros node-to-client over TCP, so the underlying node may not be reachable via the socket. This is especially common in containerized environments.

IMO, this is good enough for liveness. Now, readiness... I could see adding an additional health check for that.