[Enhancement] Improve the dab validate to include multi data source file validate

[sezal98]

In the current situation we have
"data-source-files":[
"src\Service.Tests\Multidab-config.MsSql.json"
], where we can add multi-data source files in the top-level config.

In this scenario, when running the command dab validate, we only validate the top-level config and not the files in this array.
However, when validating the top-level config, we should validate all underlying config files and provide a comprehensive output to the user defining all issues in multiple files.