Add transformer that excludes fields / inputs based on directives

[kyri-petrou]

Kind of experimental work based on transformers to allow excluding fields / input arguments based on the @GQLTag annotation.

This can be particularly helpful for schema versioning, e.g., having a /beta and a /v1 endpoint, and excluding fields from /v1 until we're happy with them without breaking compatibility in the APIs

[kyri-petrou]

Following @paulpdaniels comment here, I realised we were missing an opportunity to make the transformer work on any directive instead of just the caliban-provided annotations.

So now users can do the following:

final case class Experimental extends GQLDirective("experimental")
final case class Internal extends GQLDirective("internal")

val apiA: GraphQL[Any] = api.transform(Transformer.ExcludeDirectives(Experimental()))
val apiB: GraphQL[Any] = api.transform(Transformer.ExcludeDirectives(Experimental(), Internal()))

[kyri-petrou]

Made this an open class in Scala 3, so that users can extend it without having to add the import scala.language.adhocExtensions import. More info on open classes here

[kyri-petrou]

@paulpdaniels I'll merge this PR in, in case there is anything else you'd like me to address let me know and I can do it in a followup PR