Allow overrides in the vendor specific data type DSDL ID range

[wiboticalex]

This pull request allows overriding DSDL definitions with ones that come latest on the path list when in the "Vendor-specific messages types" range indicated at https://dronecan.github.io/Specification/5._Application_level_conventions/.

With this, I can now add DSDLs with conflicting IDs to my uavcan_vendor_specific_types directory and decode the DSDLs as expected (as was the behavior back when we were using UAVCAN V0 when the DSDL repository didn't define anything in the vendor specific range).

I think this is a reasonable approach since there are only ever 999 IDs allocated for Vendors, so the chance of collisions with existing DSDLs is relatively high as new public DSDLs become available. However, I would also be fine with a strategy such as the one called out in #30 since that would allow me to avoid the conflicting IDs in the first place.