Add create mutation for creating timeline event tags on a project

What does this MR do and why?

This MR adds a new GraphQL mutation to create timeline event tags inside a project.

These tags will be used to link them to timeline events.

Screenshots or screen recordings

How to set up and validate locally

1. Make sure you have maintainer access to a project. Copy the path of the project.
2. Use the below-mentioned mutation and input to create a timeline event tag.
3. Verify from the console that the timeline event tag was created on a project using project.find_by_full_path(<path>).incident_management_timeline_event_tags.

Mutation

mutation CreateTimelineEventTag($input: TimelineEventTagCreateInput!) {
  timelineEventTagCreate(input: $input) {
    timelineEventTag {
      id
     name
    }
    errors
  }
}

Input for mutation (Replace the project_path with the value of your project path.

{
  "input": {
    "projectPath": "flightjs/Flight",
    "name": "Test tag 0"
	}
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #373853 (closed)