Support querying Milestone by number and add helper query to get milestone by number

[chshersh]

Currently we don't have a way to query Milestone by number:

query { 
  repository(owner: "kowainik", name: "hit-on") {
    milestone(number: 1) {
      id
    }
  }
}

With the response:

{
  "data": {
    "repository": {
      "milestone": {
        "id": "MDk6TWlsZXN0b25lNDQ4MDI1Mg=="
      }
    }
  }
}

For this, we need to add a milestone connection to Repository. Similar to Issue/Issues we need to have Milestone/Milestones:

• https://github.com/kowainik/github-graphql/blob/19abc17c6fc1e03aa323fd22ea06164f79079452/src/GitHub/Repository.hs#L120-L126

Additionally, it would be nice to have a helper function to query MilestoneId similar to querying RepositoryId:

• https://github.com/kowainik/github-graphql/blob/19abc17c6fc1e03aa323fd22ea06164f79079452/src/GitHub/Query.hs#L84-L115

And add tests for this function (e.g. using the example in this issue).