github: new createIssue, getContent, getPrCommitList actions

[brig]

- task: "github"
   in:
      action: "getPrCommitList"
      apiUrl: "https://api.github.com/"
      accessToken: "XXXX"
      org: "brig"
      repo: "concord-triggers-test"
      prId: 1
    out: result

  - task: "github"
    in:
      action: "getContent"
      apiUrl: "https://api.github.com/"
      accessToken: "XXXX"
      org: "brig"
      repo: "concord-triggers-test"
      ref: "e001ce2795d72e367b7c55614dcfdc1aa8d4a8ab"
      path: "concord.yml"
    out: result

    - task: "github"
      in:
        action: "createIssue"
        apiUrl: "https://api.github.com/"
        accessToken: "XXXX"
        org: "brig"
        repo: "concord-triggers-test"
        title: "From Concord"
        body: "Some body"
        assignee: brig
        labels:
          - work in progress
          - one
      out: result