Register Checks for Group Services

[schmichael]

Followup to #6042

PR #6157 does not handle registering checks against the new ServiceCheck.Task field added in #6131.

{Register,Update,Remove}Group methods must defer check registration to TaskRunner so they can be associated with the correct Task. The associated task is required for script checks and check_restart to work.

May want to look into the misleading script check error message from #6110 at the same time.

[github-actions]

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