usb: device: next: increased RAM and flash footprint with "device_next" USB stack

[henrikbrixandersen]

Is your enhancement proposal related to a problem? Please describe.
The device_next USB stack comes with a significant increase in both RAM and flash footprint compared to the "legacy" stack. The impact is especially hard on smaller USB MCUs with e.g. 16k RAM and 64k flash (smaller STM32 parts) where the increase can prevent existing firmware applications from switching to the new stack.

Describe the solution you'd like
While the design and implementation of the device_next is much superior to the "legacy" stack, it would be nice if the RAM and flash footprint could be reduced to be more friendly towards more resource constrained MCUs.

Describe alternatives you've considered
None.

Additional context
As an example, the samples/subsys/usb/hid-mouse/ sample compiled for the frdm_k64f board increases significantly in RAM and flash footprint when built for the device_next stack, but the problem is not limited to this sample/board.

$ west build -b frdm_k64f/mk64f12 samples/subsys/usb/hid-mouse
...
Memory region         Used Size  Region Size  %age Used
           FLASH:       41016 B         1 MB      3.91%
             RAM:       12760 B       192 KB      6.49%
           SRAML:          0 GB        64 KB      0.00%
        IDT_LIST:          0 GB        32 KB      0.00%

$ west build -b frdm_k64f/mk64f12 samples/subsys/usb/hid-mouse/ -- -DCONF_FILE=usbd_next_prj.conf -DEXTRA_DTC_OVERLAY_FILE=usbd_next.overlay
...
Memory region         Used Size  Region Size  %age Used
           FLASH:       54792 B         1 MB      5.23%
             RAM:       15480 B       192 KB      7.87%
           SRAML:          0 GB        64 KB      0.00%
        IDT_LIST:          0 GB        32 KB      0.00%

[henrikbrixandersen]

These are the numbers when building the same sample with logging disabled:

$ west build -b frdm_k64f/mk64f12 samples/subsys/usb/hid-mouse -- -DCONFIG_LOG=n
Memory region         Used Size  Region Size  %age Used
           FLASH:       27288 B         1 MB      2.60%
             RAM:       10648 B       192 KB      5.42%
           SRAML:          0 GB        64 KB      0.00%
        IDT_LIST:          0 GB        32 KB      0.00%

$ west build -b frdm_k64f/mk64f12 samples/subsys/usb/hid-mouse/ -- -DCONF_FILE=usbd_next_prj.conf -DEXTRA_DTC_OVERLAY_FILE=usbd_next.overlay -DCONFIG_LOG=n
...
Memory region         Used Size  Region Size  %age Used
           FLASH:       37016 B         1 MB      3.53%
             RAM:       13368 B       192 KB      6.80%
           SRAML:          0 GB        64 KB      0.00%
        IDT_LIST:          0 GB        32 KB      0.00%