Adds a new command line for tunnel run for token file

[Cyb3r-Jak3]

Adds a new command line flag for tunnel run which allows a file to be read for the token. I've left the token command line argument with priority.

Would resolve #645

[janani-cr]

@Cyb3r-Jak3 Can you please elaborate on your need to store the tunnel token in a local file? Any details of your workflow would be helpful.

[Cyb3r-Jak3]

@janani-cr The goal would be to use docker secrets to pass the token to cloudflared. Docker secrets are passed as a file to the container. Standard practice for other images to have _FILE suffix so secrets can be used.

[rakino]

This feature is also desired for running cloudflared on Guix System or NixOS, since service definitions are done at build time and written to the store which is world-readable.

Only specifying path to a keyfile outside the store can avoid the token being included in it.

[masonfx]

This would be great to get merged. I've been looking for a more user-friendly (see: not a workaround) way to provide the tunnel token as a Docker secret.