Relay mutationWithClientMutationId does not pass forward the "extensions" field

[JCMais]

Looking at the code for mutationWithClientMutationId the extensions field is not passed forward:

graphql-relay-js/src/mutation/mutation.js

Lines 68 to 75 in 84ed000

	const {
	name,
	description,
	deprecationReason,
	inputFields,
	outputFields,
	mutateAndGetPayload,
	} = config;

The new extensions field was added on the following commit graphql/graphql-js#2097 and it's available since GraphQL v14.5.0, this new field is the recommended way to add metadata to GraphQL fields.

[jgcmarins]

related #245