Update Branch Rules query to return merge options

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Currently merge options can be queried from a project like:

query mergeOptions{
  project(fullPath:"root/test-project") {
    id
    updateDiffDiscussionPositions
    removeSourceBranchAfterMerge
  }
}

We should expose the attributes from the models created in #504648 the branch rule type.

i.e We should be able to perform queries like

query getBranchRules{
  project(fullPath:"root/test-project") {
    id
    branchRules {
      nodes {
        id
        name
        updateDiffDiscussionPositions
        removeSourceBranchAfterMerge
      }
    }
  }
}

For AllBranchesRules these should be project level settings. For other branch rules, these should be the branch specific settings.