Conditional directive application based on @tag names

[lleadbet]

Given the value of Contracts a way to split out supergraphs from a source variant, there's often use-cases where some directives may only apply to a subset of produced contracts.

Federation should enable a way to do so through a proposed new dircective @applyOnTag where, as the name implies, it'll only apply a given directive on a field if it were to be composed into the resulting contract.

As an example, deprecating fields for a given contract can be helpful, thus: field: String @applyOnTag(tag:"public", directive: "@deprecated(reason:\"test\")") (or similar).

Or for the new AuthN/Z directives, applying conditional scope checks based on the type of contract, as auth models may differ on certain contracts vs. others.

[lleadbet]

For the first example use-case, an alternative could be to accept a deprecated argument on the tag directive itself which would, during composition, apply a deprecated directive with the reason matching the deprecated string passed.

The other use-case, however, wouldn't be possible without adding a tag directive to those directives.