Lower level io methods for AssetServer.

[mintlu8]

Objective

Grant async code platform agnostic file system access to the assets folder and other AssetSources through AssetServer.

Solution

Added async methods read_stream and write_stream that can be used to read and write files as bytes or into an AsyncRead/AsyncWrite.

Example

async {
    let mut buffer = Vec::new();
    // Loads "assets/configs/config.toml"
    asset_server.read_stream("configs/config.toml", &mut buffer).await?;
    let config = toml::from_bytes(&buffer)?;
}

Testing

Added a test case for AssetServer

Issue: We currently have no tests for AssetWriter so the write function is untested. Adding testing support for AssetWriters would require a more dedicated PR.

Changelog

• Added read_stream and write_stream to AssetServer.

[mintlu8]

Writer testing is blocked on #13406.