[Enhancement] Improve the DAB Validate workflow

Currently `dab validate -c {ConfigFileName}` command validates the current mentioned config file and provides the output in the form of logs in the console and does not validate the multi data source files mentioned in this top level file.

Improvements
1. Output format to return where we need to add `dab validate -o` so we can output a junit XML file format conformant with CI/CD pipelines just like Unit Tests output structured reports.
2. Running `dab validate` on top level file should validate all multi data source files in the `"data-source-files": [....]` in the config file.

This issue would aim to improve the `dab validate ` workflow output to the users