Add an annotation to State to indicate it should always be fetched

Currently, if you have several StateIds that always need to be fetched for a stateful DoFn, you have to have a block of .readLater() calls at the beginning of each method.  It would be nicer to have an nnotation on the processElement/processTimer call params to indicate that the value should always be fetched.

Something like:

@ProcessElement

void process(@RequiredStateId("foo") ValueState foo, E element) {...}