Feature: Query String Building about apollo-link-rest

[fbartho]

Implemented via #69, #70

[zachdixon]

I've looked through the pathBuilder PR and think a query string builder would still be beneficial. I'm getting into writing all my queries and ran across the thought of having to provide the pathBuilder function every time. It doesn't seem very DRY (which may not be your goal).

On the Apollo docs page, it mentions a params option for the @rest directive:

const QUERY = gql`
  query RestData($email: String!) {
    users @rest(path: '/users/email/:email', params: { email: $email }, method: 'GET', type: 'User')

I think this is outdated as I don't see anything in the code that would support this unless I'm missing it, but something similar for the query string would be nice.

query RestData($email: String!) {
    users @rest(type: "[User]", path: '/users/email', queryParams: {email: $email})
}

This would automatically create a query string based on the queryParams and append it to the path. I think I would actually prefer this in the case of multiple @rest in the same query that may not use every param such as:

query RestData($userId: ID!, $startDate: String!, $endDate: String!) {
    user @rest(type: "User", path: '/users/:userId') {
        id
        name
        reports @rest(type: "[UserReport]", path: '/users/reports', queryParams: {startDate: $startDate, endDate: $endDate})
    }
}

If I understand correctly, this would be the equivalent using pathBuilder

let getPathBuilder = url => params => `${url}?${queryString.stringify(params)}`

const query = gql`
  query RestDate($userId: ID!, $startDate: String!, $endDate: String!, $userPath: any, $reportsPath: any) {
    user (id: $id) @rest(type: "User", pathBuilder: $userPath) {
      id
      name
      reports (startDate: $startDate, endDate: $endDate) @rest(type: "[UserReport]", pathBuilder: $reportsPath)
  }
`

client.query({
  query,
  variables: {
    id: someId,
    startDate: "05-01-2018",
    endDate: "05-31-2018",
    userPath: getPathBuilder("/user"),
    reportsPath: getPathBuilder("/user/reports")
  }
});