Improve derive macro for ExtensionDispatch

[robin-nitrokey]

• [x] provide different sets of extensions for the same backend with the different IDs
  • https://github.com/trussed-dev/trussed/pull/153
• [ ] warn if an extension is declared but not implemented
• [x] support optional backends
  • https://github.com/trussed-dev/trussed/pull/158
• [x] add skip option, e. g. for PhantomData
  • https://github.com/trussed-dev/trussed/pull/153
• [x] add no_core option to skip core requests
  • https://github.com/trussed-dev/trussed/pull/153