Make UploadableFile::get_data async for AsyncRead

[ColinKinloch]

I've been attempting to bump the version of this in obs-gitlab-runner from 0.5.0to 0.8.0.
It currently stores a map of path strings to tokio::Files for artifacts, but I've changed it to use a Vec<UploadableFile>.

My current implementation does this:

async fn get_data(&self) -> Result<Self::Data<'_>, ()> {
    let file = AsyncFile::open(&self.real_path).await.map_err(|_| {})?;
    Ok(Box::new(file.compat()))
}

I didn't make get_path async as it is used in a non async context and I'm not sure it's necessary.