Implement support frontchannel logout on openid-connect

Frontchannel logout could be implemented on Keycloak as specified in Draft: OpenID Connect Front-Channel Logout 1.0 - draft 04

Some relevant OIDC client implementations like mod_auth_oidc already support

Initialized from https://groups.google.com/g/keycloak-dev/c/GZr-qL2cquE/m/QSneckXrBQAJ

Suggested changes:

• Add enum to LoginFormsPage to represent frontchannel transition page.
• Add enum to filename mapping on keycloak/forms/login/freemarker/Templates.
• Add createFrontchannelLogoutPage() interface to LoginFormsProvider.
• Add createFrontchannelLogoutPage() implementation to FreeMarkerLoginFormsProvider.
• Implement frontchannelLogout(UserSessionModel userSession, AuthenticatedClientSessionModel clientSession) on OIDCLoginProtocol.
• Alter finishLogout(UserSessionModel userSession) on OIDCLoginProtocol.
• Show checkbox for frontchannel logout for OIDC protocol and add a text field for client attribute frontchannel_logout_url on theme/base/admin/resources/partials/client-detail.html
• Create template theme/base/login/frontchannel-logout.ftl
• Translations on theme/base/login/messages/messages_en.properties and theme/base/admin/messages/messages_en.properties

Some work done:

rhyamada/keycloak: Open Source Identity and Access Management For Modern Applications and Services (github.com)

Including frontchannel logout Tests