GuardedBy properly supporting ReadWriteLock's exclusive/shared levels

### Description of the feature request:

It would be nice if Error Prone supported specification of the desired level of locking (read or write) for ReadWriteLocks e. g. by adding `level` parameter to `@GuardedBy`:

```java
@GuardedBy(value = "lock", level = LockingLevel.WRITE)
void myMethod() { ... }
```

### Feature requests: what underlying problem are you trying to solve with this feature?

https://github.com/google/error-prone/blob/f14fb18bb05c7e9f10794771df692a42b333f18c/core/src/main/java/com/google/errorprone/bugpatterns/threadsafety/GuardedByChecker.java#L99-L102

This feature request is already expressed in a TODO in Error Prone's code itself. The point of this issue then is to identify that other people are interested in this feature and make it more easily discoverable in the repository.