(graphcache) - Allow partial schema to be passed for altered root names

[kitten]

Resolve #1378

Summary

I had some of these changes done in a forgotten branch but never proposed / opened this since we didn't have a corresponding RFC just yet, and I didn't have time to write it up.

Basically, we should allow the root names to be altered without enabling full schema awareness for exceptional cases where schema awareness is explicitly not wanted. This is rather rare with the advent of GraphQL Code Generator's built-in plugin to resolve this at build time / deploy time but it may be helpful still.

It's now possible to pass only the root names like so:

const introspection = {
  __schema: {
    queryType: {
      name: 'query_root',
    },
    mutationType: {
      name: 'mutation_root',
    },
    subscriptionType: {
      name: 'subscription_root',
    },
  },
};

cacheExchange({ schema: introspection });

Set of changes

• Make SchemaIntrospector.types optional and don't add it for partial schema introspection results
• Prevent this.schema from being set on Store when SchemaIntrospector.types is missing
• Update types in schemaPredicates.ts and fix up a test for this use-case
• Allow a partial introspection result to be passed to schema

[changeset-bot]

🦋 Changeset detected

Latest commit: ecf8610

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@urql/exchange-graphcache	Minor

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