[FR] Support ServerValue.increment

[chrylis]

The feature ServerValue.increment has been available for 4 years, and it is supported in most of the other SDKs (including the Android SDK), but it is not available in this SDK. It should be available.

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.