Add unix domain socket transport in netty 4.x via JDK16+ (#13965)

### Motivation:

Our use case requires using nio transport for unix-domain-socket (UDS)
in netty 4.x.
Although netty 5.x supports UDS, it is is not compatible with 4.x. For
us and probably many users, having to migrate to 5.x in order to use UDS
is not a viable option.

### Modification:
Add new classes to support domain UDS with NIO.

### Result:

UDS is supported when using the NIO transport when java 16+ is used.


Co-authored-by: Norman Maurer <norman_maurer@apple.com>