Add CRUD services for pipeline triggers, use CreateService in endpoints

What does this MR do and why?

Part of #421383 (closed).

This MR is based off of !130449 (comment 1635909092) and addresses bullets 1️⃣ and 2️⃣ of the following comment:

Since we are making many changes and I'm getting a bit nervous with consolidating permissions everywhere, what do you think of doing the followings:

1. In this MR we continue refining the 3 service objects.
2. In this MR we use CreateService in the 3 endpoints (Controller, REST, GraphQL) - and adjust permissions to be consistent.
3. In a separate MR we use UpdateService in the 3 endpoints (Controller, REST, GraphQL) - and adjust permissions to be consistent.
4. In a separate MR we use DestroyService in the 3 endpoints (Controller, REST, GraphQL) - and adjust permissions to be consistent.

Screenshots or screen recordings

Please see the description of !130449 (closed) for more information.

How to set up and validate locally

Please see the description of !130449 (closed) for more information.

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