fix: schema directives validation

[danielfttorres]

As a see below, the @canonical directive can be used to annotate that a specific field are canonical and can be reused by an associated API.

But when try to use @canonical directive, according to the AppSync Merged API's spec, results on:

Invalid GraphQL schema:
     Unknown directive "@canonical".

This PR fix this by adding the support for the @canonical directive.

[bboure]

Thank you @danielfttorres
We should also add the other directives (@hidden, @renamed, etc).

[danielfttorres]

Added the @hidden and @renamed directives @bboure.