Reduce number of copies in the Mesh API

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

Currently, using the Mesh API incurs a lot of unnecessary memcpy. If you have a custom mesh representation, it's even worse because you have to build a Mesh copying all your data in, and then go through all the normal Mesh overhead.

Current state of Mesh copies:

1. Mesh gets extracted which may involve a clone in `extract_render_asset`
[`extracted_assets.push((id, asset.clone()));`](https://github.com/bevyengine/bevy/blob/de888a373de666ff9d1bbdf8275e7fb625584394/crates/bevy_render/src/render_asset.rs#L296)

2. then it gets prepared into a GpuMesh, which interleave-copies the vertex data into a `Vec` in `Mesh::get_vertex_buffer_data`:
[`let mut attributes_interleaved_buffer = vec![0; vertex_count * vertex_size];`](https://github.com/bevyengine/bevy/blob/de888a373de666ff9d1bbdf8275e7fb625584394/crates/bevy_render/src/mesh/mesh/mod.rs#L469)

3. the interleave copy actually happens twice, the first time just to get the length and pre-allocate a slab 😬
[`mesh.get_vertex_buffer_data().len() as u64,`](https://github.com/bevyengine/bevy/blob/de888a373de666ff9d1bbdf8275e7fb625584394/crates/bevy_render/src/mesh/allocator.rs#L430)

and then to fill in the buffer
[`let vertex_data = mesh.get_vertex_buffer_data();`](https://github.com/bevyengine/bevy/blob/de888a373de666ff9d1bbdf8275e7fb625584394/crates/bevy_render/src/mesh/allocator.rs#L477)

4. which then gets passed by reference to wgpu `create_buffer_with_data` which immediately copies it yet again in `DeviceExt::create_buffer_init`:
[`buffer.slice(..).get_mapped_range_mut()[..unpadded_size as usize].copy_from_slice(descriptor.contents);`](https://github.com/gfx-rs/wgpu/blob/6db097694ce121f9cd927ef6dbb5608a69906760/wgpu/src/util/device.rs#L95)
which then actually gets uploaded to the gpu

## What solution would you like?

To solve 1:
Retained render world will help here

To solve 2:
introduce an api which both Mesh and custom mesh representations uses, that permits writing directly into a u8 buffer thats already mapped. this can be via a builder style api potentially.

To solve 3:
a. length can be computed without copying, b. write directly into the slab instead of creating a vec and then copying

To solve 4:
`create_buffer_init()` is a convenience function. If you want to minimize copies, then call `create_buffer()` with `mapped_at_creation: true`, then write into the mapping. - kpreid

## What alternative(s) have you considered?

Not changing it

## Additional context

i mostly want to expose an api that allows alternative mesh representations to exist easily within the renderer, and make Mesh use that api.