Add support for DeviceDiagnosis Heartbeat server per entity

[DerAndereAndi]

Right now the stack only supports heartbeats if one entity has an DeviceDiagnosis server feature with the heartbeat function.

But if a device may want to separate e.g. LPC and LPP use cases on multiple entities, each one has to have such a server feature and be able to send heartbeats. This should be made possible.