Add methods to retrieve "extra" bytes from config and interface

[cecton]

Hello @dcuddeback :wave:

Thanks for the great crate! :heart:

I submit this PR that will allow the user to retrieve the "extra" bytes from the configuration descriptor and the interface descriptor.

This is useful for me to implement a DFU crate as documented here in page 13.

[cecton]

It's not a super great fix but if the dependencies of the dependencies don't build in 1.3.0 there isn't much you can do except locking the versions :woman_shrugging: Though it's not too bad because someone who build on a more recent version of rustc will be able to compile the latest versions of everything.