Please add support for mounting FUSE filesystems

(Some of this may not be in scope for this repository; if so, please point to the appropriate place to report the rest.)

The kernel version that Lambda uses has support for mounting FUSE filesystems in a user namespace, if FUSE is enabled. (This was added in Linux 4.18.) This would enable many, many use cases, including using AWS's `mountpoint` software for S3, and other useful types of shared filesystems.

Enabling this would require:
- The ability to run a user namespace (https://github.com/aws/aws-lambda-base-images/issues/143)
- Enabling FUSE in the kernel used on Lambda
- Providing the `/dev/fuse` character device
- Optionally, providing some of the basic FUSE utilities, though users could also provide those themselves as part of their bootstrap zip.