fix: Sanitize field attributes when expanding Decode and Encode

[Huntragon]

I've added an UnsanitizedFields new type that is used in the decode derive macro to lazily sanitize the attributes on enum struct variant fields.
I've also slightly modified the encode derive macro to filter out attributes when a struct is annotated with rasn(set), which generates an inner struct.
It also includes tests for this two cases, but this required adding serde as a dev-dependency, just so I had a derive macro with custom attributes that I could use as a test (and I picked serde since that is a commonly used crate).