Title: Add file mode to Pipe listeners

Description:
Envoy can be configured with Unix domain socket listeners via the Pipe Address type. By default, these sockets are created such that only the file owner can read and write to them, and the typical umask of 0022 prevent access from group or world. We have a use case where we'd like to grant fine-grain access to individual Pipe listeners (either via group- or world-permissive configurations)

It'd be nice if Pipes could be configured individually for what mode permissions they should be created with. This would allow users to have fine-grain permissions per-listener, rather than working around it with a broad umask change.