Add support for binding to Unix sockets

[robinkar]

Fixes #6057.

Adds the possibility of passing a path to a Unix socket as the host parameter, --host unix:///path/to/tensorboard.sock, to bind TensorBoard to a Unix socket.

Motivation for features / changes

This will allow running TensorBoard securely in multi-user hosts, e.g. on HPC clusters as in my case.

Technical description of changes

As Werkzeug supports listening on Unix sockets, this mostly affects the command-line argument handling to allow passing the Unix socket path to Werkzeug.

Detailed steps to verify changes work correctly

This is easiest to test using caddy:

#!/bin/bash
socket=$(mktemp -d)/tb.sock
# Assuming the scalars_demo files have been generated already.
bazel run //tensorboard -- --logdir /tmp/scalars_demo --host unix://$socket &
caddy reverse-proxy --from :8080 --to=unix/$socket

TensorBoard should then print something like:

TensorBoard 2.21.0a0 at unix:///tmp/tmp.7e0ou10PTk/tb.sock/ (Press CTRL+C to quit)

And http://localhost:8080 should work as expected.

[boegel]

Very much +1 on this, would be great to have this reviewed/merged