AIP-122: consider warning if a string looks like it should be a resource reference but has no annotation

AIP-122 provides guidance on resource reference format, but it's difficult to identify this field, because resource references can be an arbitrary noun.

Here's an example of a string that should be a resource reference:

```
// The name of the Pub/Sub topic. Format:
// `projects/{PROJECT_ID}/topics/{TOPIC_NAME}`.
string topic = 1;
```

Perhaps we could use a heuristic here to ask the user to double-check if their field should be a resource reference?

Some possible heuristics:

- "name of" in the string, since name should be a resource name most of the time.
- "Format:" with a pathed format following it.