Add emitWithAck method to RemoteSocket class

[verycosy]

The kind of change this PR does introduce

• [ ] a bug fix
• [x] a new feature
• [ ] an update to the documentation
• [ ] a code change that improves performance
• [ ] other

Current behavior

fetchSockets() returns RemoteSocket array. But RemoteSocket does not have emitWithAck method. So currently, we can not use handy way.

New behavior

RemoteSocket instances can use emitWithAck method also.

Other information (e.g. related issues)

I just added code like emit method. It's calling BroadcastOperator's emit method.