OCS Api to GET a message from a mailbox

[miaulalala]

Is your feature request related to a problem? Please describe.

With #9691 we will provide a way to hook into the event system to get notified of incoming messages, but this event will not contain the full data of the received email, so we need a way to do so via API.

Describe the solution you'd like

Provide a OCS API that allows to GET a message.

Describe alternatives you've considered

No response

Additional context

Goes together with #9691