Don't expose implementation details in shm name

[elBoberido]

Brief feature description

Currently the SharedMemory expects a name with a leading slash. This is an implementation detail and should be removed.

Detailed information

The name of the shared memory is defined in an upper layer of the stack and then passed down to the SharedMemory object. If the implementation changes or a different mechanism is used, the leading slash might be a problem. Since this is an implementation detail of SharedMemory the slash should be added in that class.

[elBoberido]

@dkroenke just as a hint since I looked that up yesterday. In the segment_manager.inl a slash is added to the shm name. Except that, I think it's only /iceoryx_mgmt and in some tests where the slash is in the name.