Add better health check endpoint to OPAL client

[asafc]

Discussion started on slack community: https://permit-io.slack.com/archives/C01RUUYV3TP/p1652374673723599

Best thing for now is to expose the result of the health check policy to the health check endpoint, but maybe we need to expand the transaction log, due to sometimes update events fail and then succeed after retry.

cc @orishavit, @RazcoDev and @singingwolfboy let's discuss this next week.