mojo: Implement SyncGroup methods.
mojo: Implement SyncGroup methods.

MultiPart: 2/2
Change-Id: I9ff648751173c34e1ed22edc5be37e38c686cb51

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref