[RFC] Envoy as a (caching) (network) filesystem proxy

## description

Im thinking it could be very useful both in terms of edge and middle proxying if Envoy was able to proxy `http` to some (networked) filesystems - for example:

- nfs
- sshfs/fuse
- s3
- ipfs
- storj
- filecoin
- local

this suggestion would be for extensions rather than something in core.

i guess it would want to take advantage of existing stack - like kernel drivers - to utilize these filesystems, but im thinking that Envoy/extensions could be responsible for configuration/setup/teardown of the mounts - a bit like docker allows.

It would then be able to provide the Envoy routing/load balancing/network features - and potentially cache to a policy, or enforce some kind of `ACL`

it would - esp with a local driver - make Envoy more of a webserver - but at least i think in keeping with its network proxy focus.