Pluggable Egress Support

We want to make it easy for users to integrate their agent policy enforcement frameworks into the substrate Actor system. Proposal for network Egress [link](https://docs.google.com/document/d/1KmpIFu2gnqy9gp95wASgIo_vkJ_dA1DZckV8upET6bs/edit), lists the following pieces:

1. Sandbox (uVM, GVisor, etc) traffic capture to a Worker or Node local proxy.
2. Local proxy for identity, metadata injection, secure redirection to PEP
3. PEP, can be outside of Substrate.

Each of these /could/ be a point of pluggability, but this is a tradeoff -- we may not need so much flexibility at this point and we do want a default install of Substrate to come with some basic functionality for real usage.

* For (1), there are precedents on how this can be done for in Pod redirection (see mesh ecosystem). One thought is that it is less about the interception itself and more about coordination with the lifecycle of the Actor, Worker and intercept, as the latency of setup is a big factor. Given this, it probably makes sense to make a "good enough" decision and move ahead.
* There is a coupling between (2) + (3), in terms of forwarding protocol. Can we assume that (2) + (3) are coupled together in most cases so the protocol is an implementation detail OR try to standardize on a protocol to decouple (2) from (3).
* One proposal is to use HTTP CONNECT as the metadata wrapper. This does not handle UDP easily, but can take care of basic HTTP.

xref:

* https://github.com/agent-substrate/substrate/issues/126.
* https://github.com/agent-substrate/substrate/pull/338/
* https://docs.google.com/document/d/1KmpIFu2gnqy9gp95wASgIo_vkJ_dA1DZckV8upET6bs/edit