Support using client and server with existing FDs

This is the Java equivalent of grpc/grpc#3250.

I think this may just be Netty work; we could possibly enhance the epoll transport to have an address time that allows specifying the fd int directly.

Start by comparing this request with grpc/grpc#3250, then inspect the Netty and epoll transport areas mentioned in the issue. The work is complete when the Java client and server can use existing file descriptors, including support for specifying an FD directly.

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