[Feature request] Dependency injection for graphql implemataion

Is your feature request related to a problem? Please describe.

I want to use a custom graphql implementation. use cases

1. I wanna use @kitten 's graphql-web-lite or something other for a lighter bundle
2. gatsby-plugin-typegen, which is based on GCG and graphql-tools should depend on the graphql module re-exported from gatsby. Otherwise, users will get build errors due to version conflicts

Both of these can be solved today only with third-party tools such as bundlers (e.g. Webpack) or package managers (e.g. Yarn).

As usual, the JS module system cannot change its configuration by itself, so if we have base dependencies externally we configure the dependency injection by hand, instead of import impl directly.

Describe the solution you'd like

The easiest option is to accept graphql impl as an optional argument in all the functions.

const { mergeSchemas } = require('@graphql-tools/schema');

mergeSchemas({
   grapqhql: myGraphQl,
   /** ... */
});

Or a better way I think is to create a builder that allows you to customize the configuration of the function.

export function createMergeSchema({ graphql }) {
   return function mergeSchema(...) { ... }
}

export const mergeSchema = createMergeSchema({ graphql })

Describe alternatives you've considered

Transpile it and bundle the entire GCG and graphql-tools...

Start by reviewing the GraphQL Tools functions that directly import or depend on graphql, especially mergeSchemas, and compare the optional-argument and builder approaches described in the issue. Done means custom implementations such as graphql-web-lite can be supplied without bundler workarounds or graphql version-conflict build errors.