Feature Request: Add a server status topic

[apbarratt]

A topic with details of server success rate would be rather useful for quickly figuring out if a problem is likely to be leaf2mqtt or the Nissan servers.

Here's an example, though I suspect I may not fully understand the workflow of how the request and response system works here so do change as would be more appropriate :)

Where leaf represents the default root topic:

Server

Status

Topic	Type	Description
leaf/server/requestsToday	Double	The number of requests sent to Nissan's server today
leaf/server/failuresToday	Double	The number of requests sent to Nissan's server today that resulted in failure
leaf/server/successRate	Float	A percentage derived from the above values
leaf/server/lastSuccess	String	A json representation of the last topic and payload that succeeded
leaf/server/lastSuccessDateTimeUtc	Iso8601 UTC	The datetime when the last request succeeded
leaf/server/lastFailedRequest	String	A json representation of the topic and payload that failed
leaf/server/lastFailedDateTimeUtc	Iso8601 UTC	The datetime when the last request failed
leaf/server/lastRequestWasSuccessful	Boolean	True if the last request sent to the server succeeded. False otherwise
leaf/server/json	String	A json representation of all server status