buildSchema/extendSchema: add support for extensions

[IvanGoncharov]

Fixes #922

[IvanGoncharov]

Current state:

• Missing support for schema extensions
• Missing tests for Enums, Input Object, and Unions
• Missing tests for extendSchema

[sonamp]

Looks like it is not supported anymore. @IvanGoncharov could you please confirm if the extend type support is still there?

[yaacovCR]

What version are you using? Can you post a link to a reproduction?

[sonamp]

@yaacovCR Seems the issue is with graphql-tools/load, it doesn't recognize extend in schema

schema.graphql--
extend type Post {
    createdAt: String
}

Running code

const load = require('@graphql-tools/load')
const { GraphQLFileLoader } = require('@graphql-tools/graphql-file-loader')

async function main() {
const c = await load.loadSchema('/code/graphqlPoc/graphql-server/src/schema.graphql', {
    loaders: [new GraphQLFileLoader()]
})
console.log(c)
}
main()

return errors --

Error: Cannot extend type "Post" because it is not defined.
    at assertValidSDL (/code/graphqlPoc/node_modules/graphql/validation/validate.js:135:11)
    at Object.buildASTSchema (/code/graphqlPoc/node_modules/graphql/utilities/buildASTSchema.js:44:34)
    at makeExecutableSchema (/code/graphqlPoc/node_modules/@graphql-tools/schema/index.js:495:26)
    at Object.mergeSchemas (/code/graphqlPoc/node_modules/@graphql-tools/schema/index.js:532:12)
    at Object.loadSchema (/code/graphqlPoc/node_modules/@graphql-tools/load/index.js:622:29)
    at async main (/code/graphqlPoc/graphql-server/src/graph.js:5:11)

@graphql-tools/load - v7.5.13