Add utility function to transform types/directives inside GraphQLSchema

[IvanGoncharov]

TODO: write a description

Extracted from #1199

[yaacovCR]

Graphql tools has a mapSchema function that seems similar

[yaacovCR]

See https://www.graphql-tools.com/docs/schema-directives#full-mapschema-api for further discussion, you can scroll up for some examples.

code is freely available via MIT license I believe to integrate upstream or just for comparison approach

[yaacovCR]

Although my personal thought is that in terms of priorities, this has been handled in the package ecosystem and we should focus more in graphql JS on enabling custom execution and incremental delivery (defer/stream)

[Cito]

As @IvanGoncharov mentioned, this function could also be used internally to implement extendSchema and lexicographicallySortSchema.