Add ability to mutate schema in-place

[roman-vanesyan]

Currently graphql-middleware uses @graphql-tools/schema package to actually apply schema modifications to the initially provided one (via addResolversToSchema function). Here

graphql-middleware/src/middleware.ts

Line 45 in 8dc4590

updateResolversInPlace: false,

it hard-codes addResolversToSchema to always produce a new schema. I'd like to make it configurable, so I can decide whether the initial schema should be modified in-place or a new one should be generated.

This option would allow to hack around graphql-nexus/nexus#959 issue.