proxy: use middleware to manage request flow

proxy: remove duplicate error handling in New
proxy: remove routeConfigs in favor of using gorilla/mux
proxy: add proxy specific middleware
proxy: no longer need to use middleware / handler to check if valid route. Can use build in 404 mux.
internal/middleware: add cors bypass middleware

Signed-off-by: Bobby DeSimone <bobbydesimone@gmail.com>

internal/middleware/cors.go

@@ -0,0 +1,26 @@
+package middleware // import "github.com/pomerium/pomerium/internal/middleware"
+
+import (
+	"net/http"
+
+	"github.com/pomerium/pomerium/internal/telemetry/trace"
+)
+
+// CorsBypass is middleware that takes a target handler as a paramater,
+// if the request is determined to be a CORS preflight request, that handler
+// is called instead of the normal handler chain.
+func CorsBypass(target http.Handler) func(next http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			ctx, span := trace.StartSpan(r.Context(), "middleware.CorsBypass")
+			defer span.End()
+			if r.Method == http.MethodOptions &&
+				r.Header.Get("Access-Control-Request-Method") != "" &&
+				r.Header.Get("Origin") != "" {
+				target.ServeHTTP(w, r.WithContext(ctx))
+				return
+			}
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}