feat(prost-derive): skip field attribute

[maxpowa]

This change will allow Prost users to skip a field during serialization and deserialization. It also provides an optional default attribute for these fields, in the event the underlying object does not implement the Default trait.

My motivation for this is some reverse engineering effort, in which the existing datastructures are serialized in both protobuf and xml formats. I can use quick_xml/serde and ignore protobuf-only fields, but I can't ignore xml-only fields using prost, which results in duplicate data structures and more boilerplate to convert between the two types.

Usage:

#[derive(::prost::Message)]
struct Test {
    #[prost(int32, tag = "1")]
    a: i32,
    #[prost(skip)]
    b: i32,
    #[prost(skip, default = "create_foo")]
    c: Foo,
}

#[derive(Debug)]
struct Foo(i32);

pub fn create_foo() -> Foo {
    Foo(12)
}

Related: #174

---

This appears to work locally, but I can't get the full test suite to run (even with clean main branch). I'd like to add some integration tests for this, but it seems the only tests for derive currently are the existing Protobuf known-types (where this wouldn't really make sense), and a few negative test cases in the derive package.

[caspermeijn]

I like the skip option. I see how this could be useful when using prost-derive.

I am not sure how I feel about the default attribute. Why doesn't each of the skipped types implement default?

I miss tests. You could take some inspiration from tests/src/custom_debug.rs or prost-derive/src/lib.rs.

I miss documentation. The documentation of derive attributes is not great, but this new attr should at least be mentioned in README.md

[maxpowa]

I am not sure how I feel about the default attribute. Why doesn't each of the skipped types implement default?

The default attribute exists because the skipped type may be external to the implementation, and might not implement default upstream. The attribute is optional, and will rely on the type implementing Default if not defined.