Diagnostic API and service for Inputs

[patrickmann]

/nocl new feature
Relates to issue #20683

A new API and service to provide data for the Input Setup Wizard.
The new endpoint:
GET system/inputs/diagnostics/{inputId}

Returns a JSON data structure, currently only with a single object representing the message count by stream. We may add to this in future. E.g.:

{
  "stream_message_count": {
    "Default Stream": 204,
    "RandomGET": 179
  }
}

In case of no data we return an empty map:

{
  "stream_message_count": {}
}