Expand USB DFU to include more partitions

[mrrosen]

Is your feature request related to a problem? Please describe.
Currently, the implementation of USB DFU is tied to the two partitions used by mcuboot for updating application images. However, some projects might need access to more partitions over USB DFU.

Describe the solution you'd like
Discussion of possible solutions/hangups can be found in this PR: #30562

To summarize, user/app developer should have the ability to set which partitions should be exposed over the DFU interface; at present there is a refactor of the USB device stack so what that actually looks like will depend on what exposure users get to the descriptors.

Describe alternatives you've considered
See above discussion

Additional context
See above discussion