Add ability to selected inherited service desk templates from the group

What does this MR do and why?

Frontend implementation for #299084 (closed)

Currently, Service Desk is only capable of applying and listing templates from the project where it's itself enabled. With the introduction of inherited templates, we'd want to list all the available templates, from the Group level as well as the project level, and persist it on the backend.

Screenshots or screen recordings

Listing all templates

before	after

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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