[Java] - Document additionalVersions functionality (#987)

* [Java] - Document additionalVersions functionality

* changes requested

* changes as requested in pr review

* bump patch version

* changes requested

src/java/devcontainer-feature.json

 {
   "id": "java",
-  "version": "1.4.1",
+  "version": "1.5.0",
   "name": "Java (via SDKMAN!)",
   "documentationURL": "https://github.com/devcontainers/features/tree/main/src/java",
   "description": "Installs Java, SDKMAN! (if not installed), and needed dependencies.",
@@ -17,6 +17,11 @@
       "default": "latest",
       "description": "Select or enter a Java version to install"
     },
+    "additionalVersions": {
+      "type": "string",
+      "default": "",
+      "description": "Enter additional Java versions, separated by commas."
+    },
     "jdkDistro": {
       "type": "string",
       "proposals": [