Support for SAML2.0 asynchronous (front-channel) bindings for single logout

[sachiniWettasinghe]

Currently, WSO2 IS supports back-channel SLO requests with SAML SOAP binding. But Spring SAML Extension does not support SOAP binding in SLO and it supports only HTTP-Redirect and HTTP-POST bindings in SLO [1].
Therefore it is required to support asynchronous (front-channel) bindings (such as the HTTP Redirect, POST, or Artifact bindings) for single logout in WSO2 IS.

[1] https://docs.spring.io/spring-security-saml/docs/current-SNAPSHOT/reference/pdf/spring-security-saml-reference.pdf

[nadeepoornima]

Similar issue[1].

[1]. #2111