[Fix][Transform-V2] Fix multiTable SQL transform

[dybyte]

Purpose of this pull request

• Resolve ClassCastException by separating IdentityMapTransform and IdentityFlatMapTransform
• Fix test using incorrect config file that masked the issue

AbstractMultiCatalogTransform$IdentityTransform cannot be cast to org.apache.seatunnel.api.transform.SeaTunnelFlatMapTransform

Does this PR introduce any user-facing change?

No.

How was this patch tested?

TestSQL.testSQLTransformMultiTable()

Check list

• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs
• If necessary, please update incompatible-changes.md to describe the incompatibility caused by this PR.
• If you are contributing the connector code, please check that the following files are updated:
  1. Update plugin-mapping.properties and add new connector information in it
  2. Update the pom file of seatunnel-dist
  3. Add ci label in label-scope-conf
  4. Add e2e testcase in seatunnel-e2e
  5. Update connector plugin_config

[zhangshenghang]

Overall, it looks great. Could you add some unit tests?

[dybyte]

@zhangshenghang Added unit tests. PTAL, thanks.