Add @ServerEndpoint#extensions

To mirror the facility in `ServerEndpointConfig.Builder#extensions` add ability to define an array of Extensions (via class) which will be created and managed just for this endpoint. e.g.

```
public @interface ServerEndpoint {
...
public Class[] extensions() default {};
```

Start with the ServerEndpoint annotation and compare its extension support with ServerEndpointConfig.Builder#extensions. Confirm how endpoint annotations are processed and managed; done when the annotation accepts extension classes and those extensions are created and managed only for that endpoint.

Written by the indexing model from the issue text.