Migration MT-ST new field is aded in payload for export endpoint 20.13

[JimOverholt]

Pull Request summary:

Added new field [is_destination_overlay_mt ] in the payload in MT-> ST migration.

Description of changes:

class MigrationTenant(Tenant):
is_destination_overlay_mt: Optional[bool] = Field(alias="isDestinationOverlayMT")

Checklist:

• Make sure to run pre-commit before commiting 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#369