Consider supporting JEP 445

[JEP 445](https://openjdk.org/jeps/445) (Preview in JDK 21) introduces unnamed classes and instance main methods. This could be a boon to writing functions if done right changing our result from this:

```java
package com.example;

import com.google.cloud.functions.HttpFunction;
import com.google.cloud.functions.HttpRequest;
import com.google.cloud.functions.HttpResponse;

public class HelloWorld implements HttpFunction {
@Override
public void service(HttpRequest request, HttpResponse response)
throws Exception {
response.getWriter().write("Hello, World\n");
}
}
```

To something like this:

```java
import com.google.cloud.functions.FuncFramework;

void main() {
FuncFramework.http((req, res) -> response.getWriter().write("Hello, World\n"));
}
```