Timeouts for reads/writes

[jamesbornholt]

We should support timeouts for read/write operations so we don't block applications forever. These should probably be in the fs module? Currently they're baked into the prefetcher (until #120 removes them), but that seems both too low-level and also doesn't cover all the ways something could be slow, like readdir.

[jamesbornholt]

Somewhere here we need to make sure the mountpoint-s3-client crate has a way to cancel inflight requests. For example, S3GetObjectRequest right now does nothing when dropped, so the underlying CRT request just keeps on trucking: https://github.com/awslabs/mountpoint-s3/blob/main/mountpoint-s3-client/src/s3_crt_client/get_object.rs#L112