Add @ServerEndpoint#extensions

[dansiviter]

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<? extends Extension>[] extensions() default {};

[joakime]

This is something we can look into for version 5.1 (or whatever the next post Jakarta EE9 version is going to be called).

I feel we need to take a holistic approach to Extensions, as there's competing demands and usages for them (depending on the audience).

[markt-asf]

There should be an equivalent for ClientEndpoint as well.