Return alert IIDs and title in create response

What does this MR do and why?

This adds a response body to the create GitLab Alert endpoint.

Request:

{
    "title": "Memory usage exceeded threshold"
}

Response:

[
    {
        "iid": "1",
        "title": "Memory usage exceeded threshold"
    }
]

This is not considered a breaking change because the existing behaviour remains of returning a 200 response code. (Previously we returned a 200 with no response body).

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Related to #342730 (closed)