[FEATURE] Implement SAML SLO

[davidvitora]

Is your feature request related to a problem? Please describe.
Currently, the SAML implementation for Botpress doesn't provide the SLO Logout URL, which is part of the SAML specification

Describe the solution you'd like
Add a route for SLO and call strategy.logout on the current logout route

Describe alternatives you've considered
It isn't possible to create a custom router for it because we can't log out using the SDK, we also can't hook the current logout call.

Additional context
https://github.com/node-saml/passport-saml
node-saml/passport-saml#221

[davidvitora]

Available at 12.26.12