feat(auth): implement SAML SLO

[laurentlp]

This PR adds the implementation of SAML Single Log Out (SLO) to the already existing authentication strategy.

Related PR: https://github.com/botpress/botpress-pro/pull/69

This PR will require extensive testing. Here are the scenarios I thought of:

• Login when logged out of IdP
• Login when already logged in within the IdP
• Logout of Botpress and check if logged out of IdP
• Logout of IdP and check if logged out of Botpress (does not seem to work with Okta. Will have to test with another provider)
• Have multiple service providers (e.g. Botpress) logged in and check if logging out from one of them, log us out of the others. This is the SLO flow.

Note: passport-saml has some open issues that either bring constraints to using SLO or make it behave weirdly in some circustances:

• SLO does not work when authnRequestBinding: "HTTP-POST" and skipRequestCompression is set to true: Deflating request when sending POST node-saml/passport-saml#241
• Overview of edge cases that are not handled: passport-saml's IdP initiated LogoutRequest handling doesn't always close sessions node-saml/passport-saml#419
• Good explanation on how to make SLO "work": IdP-initiated SLO node-saml/passport-saml#221 (comment)

[linear]

DEV-2313 [FEATURE] Implement SAML SLO (botpress/botpress botpress/v12#1602)

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

botpress/borpress botpress/v12#1602 by @ davidvitora