[exposeField directives] Support filtering

[rbalicki2]

• right now, the generated mutation queries select all the fields selected at that location. For example, in the following field, the generated mutation query will select (at least) name and best_friend:

field Pet.BestFriendUpdater {
  set_best_friend
  name
  best_friend {
    name
  }
}

• However, if set_best_friend only changes the best_friend, we're overfetching the name!

The task

• We should add a new field to pub struct ExposeFieldDirective, something like mutation_selection_set_allow_list: Vec<SelectableFieldName> (or please, think of a better name!) which limits what fields we select.
• It should result in fewer fields being selected in the generated mutation query, and fewer fields showing up in the generated normalization AST
• note that id should always be selected

Feel free to break this up into multiple PRs.