Option to codegen impl blocks

I want to be able to define a SpacetimeType, impl From<> for a shared type, then have that impl optionally added to the codegen for use on client. The best example I can think of for this is Vec3 and Vec2 shapes, so maybe something like this:

Define your type and the impl blocks, then add #[spacetime_export] to them. This is also helpful if you want to write common functionality in the impl block and share between server + client instead of writing a separate common crate.

#[derive(SpacetimeType)]
pub struct Vec3 {
  pub x: f32,
  pub y: f32,
  pub z: f32,
}
#[spacetime_export]
impl From<Vec3> for glam::Vec3 {
    fn from(v: Vec3) -> Self {
        glam::Vec3::new(v.x, v.y, v.z)
    }
}
#[spacetime_export]
impl From<glam::Vec3> for Vec3 {
    fn from(v: glam::Vec3) -> Self {
        Self { x: v.x, y: v.y, z: v.z }
    }
}

Requested by @onx2 via the SpacetimeDB site.

Start by locating the Rust code-generation and SpacetimeType procedural-macro entry points, then trace how shared types are emitted for clients. Define what #[spacetime_export] should accept and how generated clients should include exported impl blocks, using the Vec3/Vec2 conversion example as the completion case.