[plugin] Added gradle plugin and fixed typo in docs (#769)

## Summary
Added plugin for gradle to force it to use the installed jdk version
instead of default (17).
Also fixed a small error in documentation for gradle setup

## How was it tested?
- compile
- in empty directory run `./devbox init`
- `./devbox add gradle jdk11`
- `./devbox shell`
- confirm a gradle.properties file is created with content similar to
below:
```gradle.properties
org.gradle.java.home=/nix/store/3y1sh00km4c616prn19brd3vj9ikc2h2-zulu11.48.21-ca-jdk-11.0.11
```

Author: mohsenari

docs/app/docs/devbox_examples/languages/java.md

@@ -51,7 +51,7 @@ Gradle is a popular, multi-language build tool that is commonly used with JVM pr
         mainClassName = 'hello.HelloWorld'
         jar {
             manifest {
-              /* assuming main class is in src/main/hello/HelloWorld.java */
+              /* assuming main class is in src/main/java/hello/HelloWorld.java */
                 attributes 'Main-Class': 'hello.HelloWorld'
             }
         }

plugins/gradle.json

@@ -0,0 +1,11 @@
+{
+    "name": "gradle",
+    "version": "0.0.1",
+    "match": "^(gradle|gradle_[0-9])$",
+    "readme": "You can customize which JDK gradle will use by specifying the value of `org.gradle.java.home` in gradle.properties file",
+    "shell": {
+        "init_hook": [
+            "[ -s gradle.properties ] || echo org.gradle.java.home=$JAVA_HOME >> gradle.properties"
+        ]
+    }
+}