Make the oauth2 state length configurable

[simon511000]

Authelia requires that the state must be at least be 8 characters long to ensure sufficient entropy

[yzx9]

It would be great if an environment variable could be added. May I request your attention on this?

[simon511000]

Indeed, that's a good idea, I'll take care of it!

[simon511000]

I made the OAuth2 state length configurable with a new OAUTH2_STATE_LENGTH variable, and reverted the default to 6 to preserve the current behavior.
I will create a second PR to document the configuration for Authelia once it supports adding custom user attributes (for is_admin) authelia/authelia#8081.