Add `AssetSchematic`

[MrGVSV]

Resolves #47

This PR adds support for asset schematics as defined in #47. I won't go into too much detail here, but here's an high-level overview:

• Allows defining assets directly inside a schematic definition by deriving the new AssetSchematic trait
• More examples:
  • bevy/asset_loading.rs
  • bevy/sprite_sheet.rs
  • asset_schematic.rs
• Improved bevy_proto_derive logic and organization

Here are some examples of various built-in asset schematics:

UV Sphere

(
  name: "Sphere",
  schematics: {
    "bevy_proto::custom::MaterialMeshBundle<bevy_pbr::pbr_material::StandardMaterial>": (
      mesh: Asset(UvSphere((
        radius: 1.0,
      ))),
      material: Asset((
        base_color: Rgba(
          red: 0.8,
          green: 0.7,
          blue: 0.6,
          alpha: 1.0
        ),
      )),
    )
  }
)

Sprite Sheet

(
  name: "Player",
  schematics: {
    "bevy_proto::custom::SpriteSheetBundle": (
      texture_atlas: Asset(Grid(
        texture: AssetPath("textures/rpg/chars/gabe/gabe-idle-run.png"),
        tile_size: (
          x: 24.0,
          y: 24.0
        ),
        columns: 7,
        rows: 1,
        padding: None,
        offset: None,
      )),
    )
  }
)

[MrGVSV]

This is a massive change haha but if anyone has any comments or thoughts, feel free to leave them.

I'll probably merge this pretty soon (I've been working on it for too long and getting kinda sick of it lol).

[brandon-reinhart]

This is nice. I'll probably switch to bevy_proto from my custom asset loaders after this is merged.

[MrGVSV]

Going to maybe update README to include details about this new functionality, and then I'll merge later today!