bevy_render: Add attributes and attributes_mut methods to Mesh. (…

…#3927)

# Use Case

Seems generally useful, but specifically motivated by my work on the [`bevy_datasize`](https://github.com/BGR360/bevy_datasize) crate.

For that project, I'm implementing "heap size estimators" for all of the Bevy internal types. To do this accurately for `Mesh`, I need to get the lengths of all of the mesh's attribute vectors.

Currently, in order to accomplish this, I am doing the following:

* Checking all of the attributes that are mentioned in the `Mesh` class ([see here](https://github.com/BGR360/bevy_datasize/blob/0531ec2d026085a31e937b12d5ecf4109005e737/src/builtins/render/mesh.rs#L46-L54))

* Providing the user with an option to configure additional attributes to check ([see here](https://github.com/BGR360/bevy_datasize/blob/0531ec2d026085a31e937b12d5ecf4109005e737/src/config.rs#L7-L21))

This is both overly complicated and a bit wasteful (since I have to check every attribute name that I know about in case there are attributes set for it).

crates/bevy_render/src/mesh/mesh/mod.rs

@@ -141,6 +141,20 @@ impl Mesh {
             .map(|data| &mut data.values)
     }
 
+    /// Returns an iterator that yields references to the data of each vertex attribute.
+    pub fn attributes(&self) -> impl Iterator<Item = (&str, &VertexAttributeValues)> {
+        self.attributes
+            .iter()
+            .map(|(name, values)| (name.as_ref(), values))
+    }
+
+    /// Returns an iterator that yields mutable references to the data of each vertex attribute.
+    pub fn attributes_mut(&mut self) -> impl Iterator<Item = (&str, &mut VertexAttributeValues)> {
+        self.attributes
+            .iter_mut()
+            .map(|(name, values)| (name.as_ref(), values))
+    }
+
     /// Sets the vertex indices of the mesh. They describe how triangles are constructed out of the
     /// vertex attributes and are therefore only useful for the [`PrimitiveTopology`] variants
     /// that use triangles.