Add get all pipelineTriggers graphql support

What does this MR do and why?

For #346054 (closed)

This MR adds a get all pipelineTriggers graphql endpoint.

Screenshots or screen recordings

Get all pipelineTriggers

How to set up and validate locally

1. Run gdk/gitpod server
2. Login and navigate to Settings > CI/CD > Pipeline Triggers > Expand, then create at least one trigger
3. Visit http://127.0.0.1:3000/-/graphql-explorer and query for all pipelineTriggers:

query {
  project(fullPath: "gitlab-org/gitlab-test") {
    pipelineTriggers {
      edges {
        node {
          token
          id
          canAccessProject
          description
          hasTokenExposed
          owner {
            id
          }
        }
      }
    }
  }
}

MR acceptance checklist

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