ptmgr: pass outbound_proxy to managed PT processes

When Arti launches a managed Pluggable Transport (PT) binary, the PT spec TOR_PT_PROXY requires that we set TOR_PT_PROXY in the child process's environment if an outbound proxy is configured(Socks5 Proxy (!3616 - closed) and feat: http connect proxy (!3692 - closed)).

This tells the PT to route its own outgoing connections through that proxy. Previously, the [channel] outbound_proxy setting was used for Arti's own direct connections to the Tor network, but was not forwarded to managed PT processes.

This is the followup task as mentioned in: !3616 (comment 3337076)