[Enh]: `dab validate` Output to File for CICD

The flag `dab validate -output:results.xml` extends `dab validate` to support outputting the validation results in a **machine-readable XML format** useful for CI/CD pipelines. This allows for automated validation of configuration files, where any validation errors will be written to the specified XML file for further processing or reporting just like unit test. It's okay to still output to the console.

**Sample file**

The **JUnit XML** format is widely used for reporting test results and is supported by most CI/CD tools (Jenkins, GitLab CI, Azure DevOps, etc.). You can find the JUnit XML schema reference and examples at the following link: [JUnit XML Format](https://llg.cubic.org/docs/junit/).

```xml

The data-source section is missing the required 'connection-string' field.

The entities array in the configuration is empty. Please define at least one entity.

The value 'abc' for runtime/host/mode is invalid. Expected values are 'development', 'production', or 'staging'.

```