Compose directives avaliable to plugins and in schema

[jmccaull]

Based on #1381 (reply in thread), it seems like the ability to act on compose directives at runtime was at least intended, is this the case and is this still in the works?

My use case involves adding headers with values determined by field arguments at runtime to requests made to subgraphs based on field level compose directives. The intent is to maintain the value of an argument throughout the life of a request, enabling subgraphs to optimize how they are loading data for large lists of entities. I want this to be schema driven, allowing subgraphs to add this context on their own.

Would this be possible with or better suited for Rhai scripts?