[Fix] Enhance validation for SubTaskData

[JimOverholt]

Pull Request summary:

Enhance validation model

Description of changes:

In scenario of getting subtaskdata while activating software on vManage, endpoint dataservice/device/action/status/change_partition-TASK_ID occasionally is returning dict instead of strings.

Checklist:

• Make sure to run pre-commit before committing changes
• Make sure all checks have passed
• PR description is clear and comprehensive
• Mentioned the issue that this PR solves (if applicable)
• Make sure you test the changes

[JimOverholt]

This issue was migrated from an old repository: CiscoDevNet/catalystwan#491