plugins: scaffolding + upstream plugin package

[jonmorehouse]

Here, we introduce the scaffolding around using go-plugin for upstream to

Notably, there are a few things left I'd like to do...

1.) Finish out the UpstreamManager type in the gatekeeper package. 2.) Build out internalUpstream and internalBackend types in gatekeeper package 3.) Evaluate what needs to be private and what needs to be public in the plugin/upstream package. 4.) Update makefile and build scripts to properly symlink the plugin packages 5.) Pass all options to the configure block in the plugin?

A few questions I still need to answer:

1. does the plugin/<plugin-type> package structure make sense / work? I found it a little awkward naming some things in the `upstream

2. how does an upstream.Upstream relate to an internalUpstream in the gatekeepr package? This needs to be a concrete type in upstream plugin package because a.) we need a concrete type to send over the wire and b.) we need to make sure that we don't have circular dependencies (I think a more natural place would've been to place it in the gatekeeper library, but nonetheless...

3. how does the plugin interface feel?!? Could it be simpler, possibly?

[jonmorehouse]

So this is hacky but it works end to end. Specifically here are the next steps:

• [ ] implement director (for managing multiple plugins)
• [ ] implement message passing
• [ ] implement UpstreamMatcher which listens to the Director, stores upstreams and then matches requests to the correct upstream.
• [ ] implement local LoadBalancer struct - this should talk to a load balancer plugin and handle sending messages / querying it.