Extend `--with-attribute-custom` to support more types

[jschwe]

I would like to use custom attributes to feature guard bindings based on information in the doc comment.

E.g. if the doc comment contains @since 10, I would like to generate

#[cfg(feature = "api-10")]
<the item>

Currently, add_attributes seems to only support structs, enums and unions. I would like to also have support for functions and ideally also enum variants.
Would a PR that adds function support to add_attributes() be accepted?

CC @mkroening Any technical reason your PR did not implement support for functions, or did you just not need it for your usecase?

Edit: It seems that it also is not possible / intended to add attributes during the processing of the doc comment callback, so that would also need to be changed to support my usecase.

[mkroening]

@mkroening Any technical reason your PR did not implement support for functions, or did you just not need it for your usecase?

Nope, no technical reason, I just built off the variants of --with-derive-custom, which do not support functions.

[pvdrz]

I think this is a reasonable feature, it only needs to be implemented.