Processed assets should be able to have a different file extension than unprocessed assets

From what I saw, this is not possible at the moment. Correct me if I'm wrong there or if there is already an issue tracking this.

Lets say we have a `my_image.png` that is processed with an `CompressedImageSaver` to a basis universal texture file. At the moment, the saved file inside the `imported_assets` directory sill has the `png` file extension although the file is not an valid png file anymore (because its now a basis universal texture). Of course this makes loading the files simple, since the `asset_server.load` call can just take the source path and lookup the same file inside the `imported_assets` folder. If we want to change the path of the processed asset, we need some kind of lookup table where we have an unprocessed path -> processed path mapping.

One potential way of doing this would be to store the processed file path inside the `AssetMeta` file. Lets say we have our `assets/my_image.png` and we process it with the `CompressedImageSaver` to `imported_assets/my_image.basisu` (I don't know what the proper file extension for basis universal is, lets pretend its right), we could also save the `imported_assets/my_image.png.meta` file. Every time we call `asset_server.load("my_image.png")`, we could now look up the path .meta file, and we could add a `processed_path` field there that points to the processed path ( `my_image.basisu` in this case).

I'm happy to discuss other possibilities there, this was just my initial thought how to do this. I would love to write an implementation for it, but I thought it was important to first discuss roughly what it should look like.

Start by tracing the asset processing flow around CompressedImageSaver, AssetMeta, and asset_server.load, including how files in imported_assets are located. Compare the current source-path lookup with the proposed processed_path mapping. Done should allow a processed asset to use a different extension while preserving loading from its source path.

Written by the indexing model from the issue text.