Trim the whitelisted emails

[kuboschek]

Just stumbled on this while configuring a rules' whitelist:

If I configured a@example.com, b@example.com, then both a@example.com and b@example.com (note the extra space) are authorized, but b@example.com isn't.

I suggest to either trim whitespace when checking, in this function:

https://github.com/thomseddon/traefik-forward-auth/blob/c4317b7503fb0528d002eb1e5ee43c4a37f055d0/internal/auth.go#L100C1-L108C2

Or to at least warn when parsing a rule with whitespace in an email. Happy to submit patches moving in either direction.