AuthN: Move oauthserver to extsvcauth (#75972)

* AuthN: Move oauthserver to extsvcauth

* Codeowners

pkg/services/extsvcauth/oauthserver/utils/utils.go

@@ -0,0 +1,35 @@
+package utils
+
+import (
+	"crypto/x509"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/grafana/grafana/pkg/services/authn"
+)
+
+// ParseUserIDFromSubject parses the user ID from format "user:id:<id>".
+func ParseUserIDFromSubject(subject string) (int64, error) {
+	trimmed := strings.TrimPrefix(subject, fmt.Sprintf("%s:id:", authn.NamespaceUser))
+	return strconv.ParseInt(trimmed, 10, 64)
+}
+
+// ParsePublicKeyPem parses the public key from the PEM encoded public key.
+func ParsePublicKeyPem(publicPem []byte) (any, error) {
+	block, _ := pem.Decode(publicPem)
+	if block == nil {
+		return nil, errors.New("could not decode PEM block")
+	}
+
+	switch block.Type {
+	case "PUBLIC KEY":
+		return x509.ParsePKIXPublicKey(block.Bytes)
+	case "RSA PUBLIC KEY":
+		return x509.ParsePKCS1PublicKey(block.Bytes)
+	default:
+		return nil, fmt.Errorf("unknown key type %q", block.Type)
+	}
+}