RPC plugins support (#85)

* wip

* resolve merge artifacts

* full coverage for conductor

* wire plugin conductor to main and proxy

* wip, with separate match handler

* split matching logic with another handler, add initial docs

* move parts of proxy to handlers, add tests

* add headers in to be sent to proxied url

* merged from master

* add example with docker compose

* supress excesive debug reporting 0-9 disabled in docker

* add plugin tests

* randomize test port

* lint: minor warns

* lint: err shadow

examples/plugin/main.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"context"
+	"log"
+	"net/http"
+
+	"github.com/umputun/reproxy/lib"
+)
+
+func main() {
+
+	// create demo plugin on port 1234 with two methods: HeaderThing and ErrorThing
+	// both called via RPC from reproxy core with fully formed lib.Request
+	plugin := lib.Plugin{
+		Name:    "TestPlugin",
+		Address: "plugin-example:1234",
+		Methods: []string{"HeaderThing", "ErrorThing"},
+	}
+	log.Printf("start demo plugin")
+	// Do starts the plugin listener and register with reproxy plugin conductor
+	if err := plugin.Do(context.TODO(), "http://reproxy:8081", new(Handler)); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Handler is an example of middleware handler altering headers and stastus
+type Handler struct{}
+
+// HeaderThing adds key:val header to the response
+func (h *Handler) HeaderThing(req lib.Request, res *lib.Response) (err error) {
+	log.Printf("req: %+v", req)
+	res.HeadersOut = http.Header{}
+	res.HeadersOut.Add("key", "val")
+	res.HeadersIn = http.Header{}
+	res.HeadersIn.Add("token", "something")
+	res.StatusCode = 200 // each handler has to set status code
+	return
+}
+
+// ErrorThing returns status 500 on "/fail" url. This terminated processing chain on reproxy side immediately
+func (h *Handler) ErrorThing(req lib.Request, res *lib.Response) (err error) {
+	log.Printf("req: %+v", req)
+	if req.URL == "/fail" {
+		res.StatusCode = 500
+		return
+	}
+	res.StatusCode = 200
+	return
+}