feature: basic auth0 support (#78)

* feature: basic auth0 support

* refactor: improve auth flow

* refactor: extract HttpServer config

* feature: merge HTTP API layer with Let's Encrypt

management/http_server/middleware/auth.go

@@ -0,0 +1,39 @@
+package middleware
+
+import (
+	"context"
+	"golang.org/x/oauth2"
+	"log"
+
+	"github.com/coreos/go-oidc"
+)
+
+type Authenticator struct {
+	Provider *oidc.Provider
+	Config   oauth2.Config
+	Ctx      context.Context
+}
+
+func NewAuthenticator(authDomain string, authClientId string, authClientSecret string, authCallback string) (*Authenticator, error) {
+	ctx := context.Background()
+
+	provider, err := oidc.NewProvider(ctx, "https://"+authDomain+"/")
+	if err != nil {
+		log.Printf("failed to get provider: %v", err)
+		return nil, err
+	}
+
+	conf := oauth2.Config{
+		ClientID:     authClientId,
+		ClientSecret: authClientSecret,
+		RedirectURL:  authCallback,
+		Endpoint:     provider.Endpoint(),
+		Scopes:       []string{oidc.ScopeOpenID, "profile"},
+	}
+
+	return &Authenticator{
+		Provider: provider,
+		Config:   conf,
+		Ctx:      ctx,
+	}, nil
+}