Create a tool kit for schema transformations

[brainrepo]

As explained #982 (comment) and #989 could be helpful to decouple all the schema transformation logic used in Mercurius to an external package, this can simplify the development of Mercurius plugins that which do schema transformations like Mercurius-auth.

This can also reduce the use of graphql-tools lib that is used in a bunch of Mercurius examples like https://github.com/mercurius-js/mercurius/blob/master/examples/executable-schema.js

@simoneb

[mcollina]

Absolutely!