fixup! feat: add dedicated certificate classes

lib/src/certificate.dart

@@ -4,16 +4,21 @@
 import "dart:convert";
 import "dart:typed_data";
 
+import "package:meta/meta.dart";
+
 /// Base class for certificates usable with OpenSSL.
+@immutable
 sealed class Certificate {
+  const Certificate();
+
   /// The certificate in serialized form.
   Uint8List get bytes;
 }
 
 /// A certificate in PEM format.
 final class PemCertificate extends Certificate {
   /// Creates a new [PemCertificate] from a [_certificateString].
-  PemCertificate(this._certificateString);
+  const PemCertificate(this._certificateString);
 
   final String _certificateString;
 
@@ -24,7 +29,7 @@ final class PemCertificate extends Certificate {
 /// A certificate in DER format.
 final class DerCertificate extends Certificate {
   /// Creates a new [DerCertificate] from a list of [_bytes].
-  DerCertificate(this._bytes);
+  const DerCertificate(this._bytes);
 
   final Uint8List _bytes;