private unsafe static BsonDocument SerializeKey(Jwk key)
 {
     using var bufferWriter = new PooledByteBufferWriter(1024);
     using var jsonWriter   = new Utf8JsonWriter(bufferWriter);
     jsonWriter.WriteStartObject();
     key.WriteTo(jsonWriter);
     jsonWriter.WriteEndObject();
     jsonWriter.Flush();
     return(BsonDocument.Parse(Encoding.UTF8.GetString(bufferWriter.WrittenSpan)));
 }