Sync gateway add formula

sync_gateway provides the syncing backend for couchbase mobile and is
used together with couchbase-lite-ios or android to develop syncing apps
for iOS or android.

Since the project itself is named sync_gateway I kept the name for the
formula as well.

This is a based on the PR #27051 but sync_gateway is 1.0.0 now and 
released.
Please add `version "1.0.0"` under `url`, and move `head` up under `homepage`, thanks.