feat: add custom attributes to fn items

[oberrich]

This PR adds functionality to add custom attributes to function items (proposed in #2978).
I've implemented this by extending the TypeKind used by AttributeInfo to include the Function(FunctionKind) variant (see AttributeItemKind).

Unifying the callback this way is a breaking change when --with_attribute_custom or matching on the kind field is used.

[oberrich]

I'd love to add some tests as well but I'm not sure how I would do that, can't get anything other than basic tests to run.
Some of the tests seem to be Linux only and depend on some bash script.
Would love some advice although I don't think we necessarily need tests for this since I've basically just copied the logic from @mkroening's implementation and this should just work™.

[mkroening]

You can take a look at #2866, where I added some tests along with the support for custom attributes on other items (attribute-custom.h, attribute-custom-cli.h, and their respective expected output).

[oberrich]

I've added both regular and integration tests, CI seems to pass now as well.

[oberrich]

I've added the attributes from <div bindgen attribute="..."/> annotations and included tests for them.
The PR is now ready for review.