[Request] Ability to enumerate objects by type from store for missing field handlers

[Missing field handlers](https://relay.dev/docs/guided-tour/reusing-cached-data/filling-in-missing-data/) provide a great way to normalize data fetched through different field paths.

One such use case supported today is a dropdown/table to select from a list of items (e.g. `viewer.tasks { id name}`), followed by a unit (or second page) which shows some more information about the selected item (e.g. `viewer.task(id) { id name ...TaskDescription }`). This can be supported by a handler which returns `argValues.id`, allowing the second unit/page to [partially render instantly](https://relay.dev/docs/guided-tour/reusing-cached-data/rendering-partially-cached-data/)

However, a similar use-case not supported could be a unit/page which fetches the task by another property, such as `viewer.task(name) { id name ... }`. While it would be possible to enumerate `store.getRoot().getLinkedRecord('viewer').getLinkedRecords('tasks')`, this only works when the initial dropdown fetched exactly `viewer.tasks`. In practice, it may have fetched `viewer.tasks(first: 15)`, or `viewer.tasks(completed: false)` (or `viewer.completed_tasks`). Regardless of the fields involved, the relay store will contain a list of `Task` objects keyed by their IDs. If there was a function `store.getAllByType('Task')`, we could enumerate them (albeit somewhat inefficiently, but for a small number of tasks this is fine; perhaps a more robust solution would allow a listener for any Task being updated to maintain this lookup map?) and find the task with the matching name.

I think it's fairly common that the id is not the only method to identify/lookup items (ex. the URL may often contain a friendlier name), so it'd be valuable to lookup data from the store in other ways for missing field handlers. From what I can tell, there is no function exposed to do this today.