Add Heartbeat() to plugin interface

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fixes #49 (closed)

Define the interface for the new Heartbeat RPC.

Changes

• extend protobuf protocol with Heartbeat RPC
• extend related interfaces

Hi @ajwalker,

I would like to start the implementation in the fleeting plugin for AWS to we need an interface to be defined.

Sven Geisler <sven.geisler@mercedes-benz.com>, Mercedes-AMG GmbH, Provider Information