Ability to join a UDP socket to multicast group

### Ktor Version
1.1.2

### Ktor Engine Used(client or server and name)
ktor-network
### JVM Version, Operating System and Relevant Context
openjdk 11.0.1, Windows, Linux

### Feedback

We very lacks ability to join a UDP socket to multicast IPv6 group.

Please, consider adding this feature.

Start in the ktor-network module by locating the UDP socket API and any existing multicast support. Determine how joining an IPv6 multicast group should be exposed and behave on Windows and Linux, then verify the completed feature with coverage for joining and receiving from such a group.

Written by the indexing model from the issue text.