Adding get_size_on_remote to the Transport interface

[GeigerJ2]

As noted during discussions with the core office, @agoscinski, @khsrali, @unkcpz, on #6584:
It would be nice if the Transport could implement a method such as get_size or get_size_on_remote to which a path can be provided (as well as additional endpoints, if we think of more in the future).

In #6584, some custom logic to be added because FirecREST doesn't support arbitrary command execution and the aiida-firecrest transport/scheduler plugin therefore doesn't have an exec_command_wait method. However, this seems to be more a concern of the transport. As this change would require adding to the abstract interface of Transport, as well as implementing the methods it in its subclasses, just putting out the issue here as a reminder for the future for now.

[khsrali]

transport.du() 🥇