feat: add support for README in FUSE mode

[enocom]

This PR adds:

• CLI flags for fuse and fuse-temp-dir
• A check if the OS supports FUSE
• a FUSE server that serves a README
• Support for closing the FUSE server

There are four FUSE libraries for Go:

bazil.org/fuse -- a solid library, pure Go implementation, but Linux only.
github.com/jacobsa/fuse -- a fork of the library above. Linux and Darwin are supported.
github.com/hanwen/go-fuse -- pure Go implementation, Linux and Darwin are supported.
github.com/winfsp/cgofuse -- broad OS support (Linux, macOS, Windows), but requires CGO.
What we want:

Broad OS support -- especially Linux and macOS.
No CGO -- we compile cross-platform and static binaries. CGO would introduce a lot of complexity into our build process that we'd rather avoid.
Well maintained and active project.
By the criteria above, that leaves us with:

github.com/jacobsa/fuse and
github.com/hanwen/go-fuse

Option 2 has better performance, so we are moving forward with go-fuse.

The next PR(s) will include:

• returning a symlink to a Unix domain socket that is connected to the Cloud SQL instance
• (maybe) support for "warming up" instance connection names in FUSE mode