Add group credentials inventory tags to REST API

What does this MR do and why?

Add Grape tags to group credentials inventory endpoints.

References

https://docs.google.com/spreadsheets/d/1UFnfUEHKRHWn3tLwFfhkBpYx_Pb9AQHjGJRPJCC6EY8/edit?gid=838451730#gid=838451730

MR acceptance checklist

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