FR: AutoClosable AttachedContext

Today, if you want to use [`io.grpc.Context.attach()`](https://grpc.io/grpc-java/javadoc/io/grpc/Context.html#attach--), you have to provide your own `try`/`finally` block. In order to take advantage of try-with-resources, if there were a `AttachedContext` that implemented `java.lang.AutoClosable`, then you could have

try (AttachedContext ac = new AttachedContext(context)) {
…
}

and omit the `finally` block.