Ability to use SOCK_DGRAM sockets

I want to route UDP traffic to a Unix Domain Socket, but I can't do that right now because there is no way to open the socket in SOCK_DGRAM mode. I think it would be a very useful feature if it cloud do that. Is there any chance that you will be able to support it in the near future?

The issue names no files, tests, or entry points. Start by locating Envoy's Unix Domain Socket configuration and socket-creation paths, then determine the expected behavior for SOCK_DGRAM support and add coverage for routing UDP traffic through a Unix Domain Socket.

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