Support variables in @requires directives #9583

Is your feature request related to a problem? Please describe.

There's a little detail on this page; Contribute and Reference Entity Fields, that caught me a bit by surprise:

Starting in Federation v2.1.2, the @\requires directive can include fields that take arguments, like so:
...
The following rules apply:
...

• Each specified argument value is static; the router always provides the same value.

We have been using the @requires directive to implement a Server-Driven-UI subgraph. The pattern has proven quite useful for creating a nice delineation between our "Data Layer" and our "UI Layer".

In implementing this, we've run into a roadblock. In our specific use-case we have parameterized fields, like icon(size: ImageSize) {...} where our clients pass in some parameters for the image size they need. While we can use the @requires directive to statically request a certain image size, this doesn't allow our clients to pass the desired image size to the field for the UI Component which happens to reference the data-layer icon via @requires (a detail hidden from the consumers of the API).

This particular use-case we can likely work around by accepting the parameters on our UI field, using @requires to access a raw url and formatting it with the size parameters in the SDUI subgraph - this case makes sense to be a UI concern.

There are; however, other upcoming use-cases where this will be a hard blocker. The one that jumps to mind is anything involving pagination. To return a page '3' of some list of widgets we'd have to do something like the following, which would be untenable.

type WidgetListUI {
  widgetData: WidgetList! @external
  widgets(cursor: String, limit: Int): [WidgetItemUI!] @requires(fields: "widgetData(pagination: {cursor: \"\", limit: 100_000}) { widgets { name ....} }")
}

limit: 100_000 ... hopefully you see how this is problematic 😆

Describe the solution you'd like

It would be ideal if we could define arguments on a field that uses @requires and reference those variables inside the @requires

type WidgetListUI {
  widgetData: WidgetList! @external
  widgets(pagination: Pagination!): [WidgetItemUI!] @requires(fields: "widgetData(pagination: $pagination) { widgets { name ....} }")
}

Describe alternatives you've considered

We've considered alternative ways of organizing our schema, we have our hands tied a little due to legacy schema, but also any alternatives provide a less than optimal schema for our consumers.