Aux-N persistent assignments support

The stack supports now basic Aux-N assignments for the VT. I think we want to have the stack save these assignments persistently and load them in on next startup. This is also what the standard requires/wants from the working set.

To make this implementation not depend on any particular storage type, my idea was to make the storage part abstract and let the application decide on the (platform specific) implementation. We can possibly also provide some of these implementations - For example the Linux and/or Windows filesystem, ESP flash storage, etc...

Related TODOs: Auxiliary assignments should be loaded (e.g. from file/flash/cloud etc...) and then published to the VT after it loaded the object pool: https://github.com/ad3154/ISO11783-CAN-Stack/blob/22094dd22a460e46d8778d9a40c26ac5d529a3d0/isobus/src/isobus_virtual_terminal_client.cpp#L2318 After these auxiliary assignments have been published to the VT, we expect to get an approval/denial. If the preferred assignments command is approved (and therefore the assignments are implemented) by the VT, we can have to load them in on our end as well: https://github.com/ad3154/ISO11783-CAN-Stack/blob/22094dd22a460e46d8778d9a40c26ac5d529a3d0/isobus/src/isobus_virtual_terminal_client.cpp#L2863 If the VT operator assigns an auxiliary input to our function, and the VT wants us to save this assignment persistently, we have to store it as well. (e.g. to file/flash/cloud etc...) https://github.com/ad3154/ISO11783-CAN-Stack/blob/22094dd22a460e46d8778d9a40c26ac5d529a3d0/isobus/src/isobus_virtual_terminal_client.cpp#L2896

Extension of #123

Open-Agriculture / AgIsoStack-plus-plus

Aux-N persistent assignments support #136