Add keycloak alias guard extension (#776)

Closes #775

Signed-off-by: Александр Колосов <dnwSilver@gmail.com>

Author: dnwSilver

extensions/keycloak-alias-guard.json

@@ -0,0 +1,6 @@
+{
+  "name": "Alias Guard",
+  "description": "User Profile validator and event listener that canonicalizes email addresses (Gmail dots, +tags) and blocks registration or email change when the mailbox is already taken. One inbox — one account.",
+  "website": "https://github.com/it-perspectives/keycloak-alias-guard"
+}
+