Implementation of the execute method in the PreparedStatementImpl class

[echo-42]

Issue: #62

Implementation of "execute" method in the PreparedStatementImpl.
Some tools want to see this method. Since the implementation is already in StatementImpl, in my opinion it is fair to add it to PreparedStatementImpl.
I did this by analogy.

[dai-chen]

@echo-42 Thanks for your PR. Could you please add some unit test for your changes?

[echo-42]

@echo-42 Thanks for your PR. Could you please add some unit test for your changes?

I did not find an existing test for PreparedStatement. I had to add a new test class.

[dai-chen]

@echo-42 Hmm, it seems missing. I can only find this one: https://github.com/opendistro-for-elasticsearch/sql-jdbc/blob/master/src/test/java/com/amazon/opendistroforelasticsearch/jdbc/StatementTests.java. Thanks for adding a new one. Will review your PR with others.

[dai-chen]

Approved and waiting a little while for others to review. Will merge if looking good. Thanks for your contribution!

[echo-42]

thanks =)