@Override
    public void start() throws ServerException {
        try {
            Optional.ofNullable(sslContext).ifPresent(DynamicSslContext::start);
            server = nettyServerBuilder.build();
            server.start();
        } catch (IOException e) {
            throw new GRPCServerException(e.getMessage(), e);
        }
    }
Require for `server.crt` and `server.pem` for open ssl at server side.

@param certChainFile  `server.crt` file
@param privateKeyFile `server.pem` file
oap-server/server-library/library-server/src/main/java/org/apache/skywalking/oap/server/library/server/grpc/GRPCServer.java