[java-firestore] Add a snippet demonstrating a stale read.

We need a Firestore snippet that demonstrates how to set read options to perform a stale read. A stale read can improve latency for request that don't require strong consistency.

File to update:

https://github.com/googleapis/java-firestore/blob/main/samples/snippets/src/main/java/com/example/firestore/snippets/ManageDataSnippets.java