Add pipelineTriggersCreate GraphQL mutation

What does this MR do and why?

Part of #346054 (closed)

This MR adds a new mutation to CREATE a new pipelineTrigger.

Related MRs:

• Get all pipeline triggers: !125887 (merged)
• Update pipeline trigger: !126537 (merged)
• Delete pipeline trigger: !126540 (merged)

Screenshots or screen recordings

How to set up and validate locally

1. Run gdk/gitpod server
2. Visit http://127.0.0.1:3000/-/graphql-explorer and create a new pipelineTrigger using the below mutation

mutation {
  pipelineTriggerCreate(
    input: {projectPath: "gitlab-org/gitlab-shell", description: "This is my second new trigger!"}
  ) {
    pipelineTrigger {
      id
      description
      lastUsed
      owner {
        id
      }
      hasTokenExposed
      token
      canAccessProject 
    }
    errors
  }
}

MR acceptance checklist

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