support for front-channel SAML SingleLogoutService profiles

[hauntingEcho]

Description:
When a user has an SSO session active at an SP and logs out from WSO2, they should also be logged out of those SPs per the SAML2 Single Logout specification. I have not had reason to check SOAP profile support, but neither POST nor Redirect profiles are used.

Copied from the plugin repo as this one seems to get more attention:
wso2-extensions/identity-inbound-auth-saml#167

Suggested Labels:
Affected/5.4.0, Component/SAML,

OS, DB, other environment details and versions:
Debian stable (9), WSO2 IS 5.4.0, FireFox LTS (52)

Steps to reproduce:

• set up a default installation of WSO2 IS 5.4.0
• set up a Service Provider with a SAML metadata file including SingleLogoutService endpoints with a binding of either "urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST" or "urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect"
• set up a user in both the SP and WSO2, with access to log in to the WSO2 dashboard
• use a request from that service provider to log in to WSO2
• navigate to the WSO2 dashboard
• open dev tools to watch the request
• logout (via the WSO2 dashboard)
• see that no SLO request was made to the service provider
• navigate to the service provider and see that you are still authenticated

[thanujalk]

@hauntingEcho We have captured the same improvement in #4038 and it will available in the IS 5.8.0-M26.