ProcessManager refactoring

## Brief feature description

The `ProcessManager` does not only handle the processes but also handles the requests from the posh runtime. This makes is difficult to test and the process handling needs to be untangled.

## Detailed information

The code from `RouDi::processMessage` should be refactored to something like `CmdHandler`. This `CmdHandler` would take care of all the communication, receiving request and sending responses. The refactored `ProcessManager` would only take care of the process handling and be called by the `CmdHandler`.

This could be done in combination with #1133

Start with RouDi::processMessage and trace how ProcessManager currently handles process work and requests from the posh runtime. Review the related scope in #1133 before deciding on the CmdHandler boundary. Done means communication is handled separately while ProcessManager focuses only on process handling, with tests made easier to isolate.

Written by the indexing model from the issue text.