Add the metadata module for Sharding-JDBC

[tristaZero]

The usage of metadata：

1. Append derived Columns for 'order by' and 'group by'.

2. Support insert statement without columns.

3. Generate primary key for insert statement without columns.

4. Get the information of Sharding datasources or default datasource from metadata for 'desc table;'.

5. Get the information of Sharding datasources or default datasource from metadata for 'show create table;'.

6. Get the information of Sharding datasources or default datasource from metadata for 'show tables;'.

7. Get the amount of columns from metadata for prepare statements in Sharding-Proxy.