Improve derive macro for ExtensionDispatch

[robin-nitrokey]

• provide different sets of extensions for the same backend with the different IDs
  • derive: Add options for ExtensionDispatch #153
• warn if an extension is declared but not implemented
• support optional backends
  • Add OptionalBackend wrapper #158
• add skip option, e. g. for PhantomData
  • derive: Add options for ExtensionDispatch #153
• add no_core option to skip core requests
  • derive: Add options for ExtensionDispatch #153