[java-firestore] FR: Add precondition support to 'set' methods

The 'update' methods of UpdateBuilder and DocumentReference support a Precondition parameter, while 'set' methods do not, even though they use the same underlying server API, which supports preconditions.

I suggest adding precondition support to 'set' methods as well, as that would be useful for cases where you want to replace a document, unless it has been modified elsewhere since you loaded it.

I have implemented a POC of this feature, and tested that it works. I would be happy to submit a PR.