Add the backchannel logout event

Description

Added SSE-Event when logged out

test environment: build an image and run ocis_keycloak
test case 1: Login as admin to ocis.owncloud.test, and open one more tab in a browser. Watching the sse in a browser console. After logout, the browser has get an event. The message format: type: backchannel-logout data:{"userid":"93277962-0054-47d5-9717-fed84f26b9e7","timestamp":"seconds:1718982662 nanos:826943110 "}

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Technical debt
[ ] Tests only (no source changes)