Custom skin example

[dyc3]

• bare minimum to do custom skins on player entities
• add a CustomSkin component

Objective

This PR aims to make it a little bit easier to create player entities with custom skins. This is common for things like NPCs.

closes #460

Solution

• Added a new example.
• Ideally, we could provide the CustomSkin component inside valence itself, maybe in a new valence_util crate?

[rj00a]

Instead of a new component, perhaps we should use a custom world query with convenience methods to avoid duplicating data.

[dyc3]

@rj00a Would you mind elaborating on your idea there? I might have some spare time soon to fix this up.

[rj00a]

I was thinking something like this:

#[derive(WorldQuery)]
#[world_query(mutable)]
pub struct SkinQuery {
    pub properties: &'static mut Properties,
}

impl SkinQueryItem<'_> {
    pub fn skin(&self) {}
    pub fn cape(&self) {}
    pub fn set_skin(&mut self) {}
    pub fn set_cape(&mut self) {}
}

impl SkinQueryReadOnlyItem<'_> {
    pub fn skin(&self) {}
    pub fn cape(&self) {}
}

[rj00a]

wait, we can just make those methods on Properties.