feat(apigateay): add OIDC and basic auth support

- when setting an OIDC issuer URL the defaulter will fetch and set
  authorization and token endpoints
- basic auth allows to use either inline hashed credentials or plaintext
  credentials from a secret that are automatically hashed
- finish TLS support for API & dashboard listeners

internal/oidc/discovery.go

@@ -0,0 +1,60 @@
+/*
+Copyright 2025 Peter Kurfer.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package oidc
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"strings"
+)
+
+var ErrUnexpectedStatusCode = errors.New("unexpected status code")
+
+type DiscoveryDocument struct {
+	AuthorizationEndpoint string `json:"authorization_endpoint"`
+	TokenEndpoint         string `json:"token_endpoint"`
+}
+
+func IssuerConfiguration(ctx context.Context, issuerUrl string) (dd DiscoveryDocument, err error) {
+	const oidcDiscoveryEndpoint = "/.well-known/openid-configuration"
+	if !strings.HasSuffix(issuerUrl, oidcDiscoveryEndpoint) {
+		issuerUrl = strings.TrimSuffix(issuerUrl, "/") + oidcDiscoveryEndpoint
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, issuerUrl, nil)
+	if err != nil {
+		return dd, err
+	}
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return dd, err
+	}
+
+	defer func() {
+		err = errors.Join(err, resp.Body.Close())
+	}()
+
+	if resp.StatusCode != http.StatusOK {
+		return dd, fmt.Errorf("%w: %d - %s", ErrUnexpectedStatusCode, resp.StatusCode, resp.Status)
+	}
+
+	return dd, json.NewDecoder(resp.Body).Decode(&dd)
+}