Let custom ObjectProvider implementations declare a single method only

[jhoeller]

Historically, ObjectProvider comes with several default methods already but still requires concrete implementations to declare four methods. For unit testing purposes, it would be great to just declare the concrete method that a caller actually needs (such as getObject() or getIfAvailable()), or to provide a stream() implementation with all other ObjectProvider methods reacting to it, e.g.

		new ObjectProvider<MyBean>() {
			@Override
			public Stream<MyBean> stream() {
				return Stream.of(bean1, bean2);
			}
		};

or

		new ObjectProvider<MyBean>() {
			@Override
			public Stream<MyBean> stream() {
				return Stream.empty();
			}
		};

As of 6.2, a complete set of default methods in ObjectProvider itself enables all kinds of such custom usage.