feat: add custom attributes to `fn` and `extern`-block items

[rich-ayr]

Currently you are able to add custom attributes to Struct, Enum and Union but not in other places via ParseCallbacks::add_attributes but not to fn items or extern blocks.

This would allow me to use #[export_tokens] on those items and use the tokens in different macros.

See also #2866

[rich-ayr]

Closing in favor of older issue for tracking.