feat: keycloak config

Closes https://git.coopcloud.tech/coop-cloud/matrix-synapse/issues/18.

homeserver.yaml.tmpl

@@ -1896,6 +1896,19 @@ oidc_providers:
   #    - attribute: userGroup
   #      value: "synapseUsers"
 
+  {{ if eq (env "KEYCLOAK_ENABLED") "1" }}
+  - idp_id: keycloak
+    idp_name: {{ env "KEYCLOAK_NAME" }}
+    issuer: "{{ env "KEYCLOAK_URL" }}"
+    client_id: "{{ env "KEYCLOAK_CLIENT_ID" }}"
+    client_secret: "{{ secret "keycloak_client_secret" }}"
+    scopes: ["openid", "profile"]
+    user_mapping_provider:
+      config:
+        localpart_template: "{{ user.preferred_username }}"
+        display_name_template: "{{ user.name }}"
+  {{ end }}
+
 
 # Enable Central Authentication Service (CAS) for registration and login.
 #