Consul Config Sourcer Plugin

[paladin-devops]

This PR adds a new config sourcer plugin to support dynamic configuration from the Consul KV data store.

Example config source settings:
waypoint config source-set -type=consul -config=address=$CONSUL_HTTP_ADDR -config=token=$CONSUL_HTTP_TOKEN

Example waypoint.hcl snippet using this plugin:

  config {
    env = {
      "TEST" = dynamic("consul", {
        key  = "paladin"
      })
    }
  }

This plugin was authored by @mkeeler! 🎉

[catsby]

A lot of nit-pick things, most of which are inconsequential but a few I'd like your thoughts on before 👍

[briancain]

Sweet!

[catsby]

👍 LGTM