refactor(xwiki): move extension installer logic into static Groovy file and switch to plugins dict

- Added 'plugins' section in config/main.yml to declare enabled extensions in a structured way
- Introduced new static file 'files/extension_installer_b64.groovy' that decodes Base64 JSON of requested plugins
- Simplified 04_extensions.yml: now builds installer code from static file and removed hardcoded OIDC/LDAP checks
- Dropped redundant XWIKI_EXT_* variables in vars/main.yml
- Added XWIKI_PLUGINS fact to collect enabled plugin items from config/main.yml

This refactor makes extension installation more generic, easier to unit test, and extendable beyond OIDC/LDAP.

See: https://chatgpt.com/share/68ca25e3-cbc4-800f-a45e-2b152369811a

roles/web-app-xwiki/config/main.yml

@@ -39,3 +39,25 @@ rbac:
   roles: {}
 ldap:
   local_enabled: true # Allows local login if LDAP is down
+
+plugins:
+  oidc:
+    enabled: true
+    items:
+      - id: "org.xwiki.contrib.oidc:oidc-authenticator"
+        version: "2.19.2"
+#      - id: "org.xwiki.contrib.oidc:oidc-authenticator-ui"
+#        version: ""          # let EM choose a compatible version
+
+  ldap:
+    enabled: false
+    items:
+      - id: "org.xwiki.contrib.ldap:ldap-authenticator"
+        version: "9.15.7"
+
+  # Example for Matomo
+  matomo:
+    enabled: false
+    items:
+      - id: "org.xwiki.contrib:matomo"
+        version: "1.0"