Simplified Jib plugin extension configuration

When using a single extension that doesn't define an extension-specific config, the user currently has to put quite some block into the build file.

- Maven
```xml


com.example.Foo


```

- Gradle
```gradle
pluginExtensions {
pluginExtension {
implementation = 'com.example.Foo'
}
}
```

It'd be nice if they could do

```xml
com.example.Foo
```

```gradle
pluginExtensions = 'com.example.Foo'
```

or the following if the above doesn't make sense:

```xml


com.example.Foo
com.example.Bar

```
```gradle
pluginExtensions = ['com.example.Foo']
```