feat: allow after build hook to modify the schema

[hemache]

related to #959

usage example:

import { applyMiddleware } from 'graphql-middleware'
import { yupMiddleware as YupMiddleware } from 'graphql-yup-middleware'
...

const yupMiddleware = YupMiddleware()

export const middlewarePlugin = (): NexusPlugin => {
  return plugin({
    name: "MiddlewaresPlugin",
    onAfterBuild(schema) {
      const { schema: ret } = applyMiddleware(schema, yupMiddleware)
      return ret
    },
  })
}

const schema = makeSchema({
  ...
  plugins: [middlewarePlugin()],
})

[tgriesser]

Interesting change, seems reasonable, mind adding a test to cover it?

[tgriesser]

Actually, I think we want to add a new hook here, onBeforeSchemaBuild which takes the config that will be passed to new GraphQLSchema, and can modify that, which is better than needing to build a whole new schema.