fix: do not serialize unnecessary fields (#1426)

Do not serialize java.util.regex.Pattern and executor fields in client
side statement. This caused an issue with Java 16 compilation, where an
IllegalAccessException is thrown during JSON serialization of such
fields.

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ClientSideStatementImpl.java

@@ -131,10 +131,10 @@ public String getMessage() {
   private ClientSideSetStatementImpl setStatement;
 
   /** The compiled regex pattern for recognizing this statement. */
-  private Pattern pattern;
+  private transient Pattern pattern;
 
   /** A reference to the executor that should be used. */
-  private ClientSideStatementExecutor executor;
+  private transient ClientSideStatementExecutor executor;
 
   /**
    * Compiles this {@link ClientSideStatementImpl}. Throws a {@link CompileException} if the