Provide a framework proxy setup

[RoganDawes]

The framework could provide a "default" proxy handler on all outbound connections, so that upstream proxies only need to be configured in one place for simple pipelines. This would also enable framework services such as #5 to retrieve certificate chains, even when a proxy is required to reach the destination.

Alternatively, pipelines may configure their own branch-specific proxy configurations, and the framework should detect this and get out of the way. This gets a little more complicated when considering the SubChannel, as finding instances of a ProxyHandler handler on a different pipeline means you have to look up that pipeline first.

If the framework proxy configuration is flexible enough, e.g. network/mask -> proxy mapping, you should be able to avoid pipeline-specific proxies, I guess.