Macro to generate MeshVertexAttributeId from call site info

## What problem does this solve or what need does it fill?

Currently the recommended way to get a unique `MeshVertexAttributeId` seems to be to hardcode an arbitrary number, which is clunky and error-prone (human-picked numbers are generally not very random). We can do better than this, with a macro that generates a unique id using the `file!()`, `line!()` and `column!()` macros in Rust. That would make it essentially impossible for an id collision,

## What solution would you like?

Currently, in the [`custom_vertex_attribute` example](https://github.com/bevyengine/bevy/blob/87722d135fcc7e6058aa4710e7fe5be5f2a6e594/examples/shader/custom_vertex_attribute.rs#L27), the id for the BlendColor attribute is hardcoded to 988540917. A more ergonomic and less error-prone way to do this might look like the following:

```rs
const ATTRIBUTE_BLEND_COLOR: MeshVertexAttribute =
MeshVertexAttribute::new("BlendColor", id_from_callsite!(), VertexFormat::Float32x4);
```

## What alternative(s) have you considered?

This seems like a good use case for a UUID, but doing that automatically would make the ordering of `MeshVertexAttribute`s non-deterministic, which could lead to extremely difficult to find bugs.

## Additional context

[`examples/custom_vertex_attribute`](https://github.com/bevyengine/bevy/blob/latest/examples/shader/custom_vertex_attribute.rs)
[`MeshVertexAttributeId`](https://docs.rs/bevy/latest/bevy/render/mesh/struct.MeshVertexAttributeId.html)

Note: need suggestions for a good name for this macro.