Support for directive deviation checks in Schema Checks/Linting

[lleadbet]

Context

Federation allows for custom executable directives to exist in the supergraph so long as all subgraphs agree upon a shared definition. This means that if a single subgraph either omits or changes the definition for a custom directive, it is removed from the resulting supergraph, potentially impacting client operations.

This feels like the ideal situation for either schema checks or linting to resolve as it can either result in a warning via linting (or error if configured), or as an error if the resulting supergraph would break client usage.