add engagement model to app file

[maorfr]

supporting the proposal in https://gitlab.cee.redhat.com/app-sre/contract/-/merge_requests/109

adding engagementModel to app files will help determine the engagement level with each different service.

this attribute can be added to either child apps or parent apps, illustrating the engagement level per component if needed.

starting out with this attribute as optional so we don't create a breaking change that requires every instance to add this attribute to all app files. we can make it mandatory down the line, or not. perhaps the default is Standard, unless otherwise specified.

[rporres]

Do we need this if the status of the onboarding is different? The related contract MR mentions a different onboarding status.