[FEATURE] Support implicit flow

Is your feature request related to a problem? Please describe. I tried to setup OIDC for guacamole, which failed as guacamole only support implicit flow

Describe the solution you'd like Tiny Auth should support implicit flow, and the user should be able to select which flow to use per application (probably using an environment variable for each service like the rest of the conf)

Describe alternatives you've considered No OIDC on guacamole (not the end of the world as there's the standard authentification available)