Add support for custom equality functions in Eq/PartialEq

[fredszaq]

This PR depends on #532 (I will mark this PR as non-draft once #532 is merged; for now, you will see changes from both PRs in the diff).

Synopsis

This adds the possibility to use custom functions to perform the equality check in PartialEq/Eq derives. This is quite useful when wrapping types without a PartialEq/Eq impl

Solution

This builds on #532 and reuse the FieldAttributes for Hash, I moved the struct to utils::attr and renamed it WithOrSkip.

I removed fields types that have a with attribute from the automatically added where Type: Eq bound automatically generated by the Eq derive. This seems like the thing to do as if you're using a custom function it most probably means the field doesn't implement Eq.

While this is not part of the roadmap for PartialEq, defined in #163 this is something that will be quite useful for people migrating form derivative as this was something supported by this crate

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)