feat: Add file: arg to auth_key directive

[sekunho]

Hey 👋

This adds the ability for caddy-tailscale to set the auth_key directive using the provided file's contents. The file:<PATH> syntax is akin to what is being used in the CLI.

tailscale {
  auth_key "file:/path/to/authkey"
}

If it's not prefixed with file:, it falls back to the default behavior where it treats it as the actual authkey value.

[sekunho]

I'm not sure where else to put this but this is only relevant for the test. 😅

[willnorris]

Hey, thanks for the contribution! It turns out this actually isn't need though, since caddy provides this support natively as a global placeholder. This syntax is only slightly different from what you have. Try this:

{
  tailscale {
    ephemeral
    auth_key {file./tmp/key}
  }
}

:80 {
  bind tailscale/
  respond OK
}

[sekunho]

Oh wow wait I had no idea this was a thing. Thank you!!