feat(api-graphql): add model index query support

[iartemiev]

Description of changes

Runtime implementation for this feature.

// Given this gen2 schema
const schema = a.schema({
  Post: a
    .model({
      title: a.string().required(),
      description: a.string(),
      viewCount: a.integer(),
      updatedAt: a.string(),
    })
    .secondaryIndexes([ a.index('title').sortKeys(['viewCount']) ])
});

// We can now perform the following index query using Amplify JS:
const { data: posts } = await client.models.Post.listByTitleAndViewCount(
  { title: 'My Post', viewCount: {gt: 4} }
);

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[stocaaro]

I'm torn on this one. There are two lines I'm tempted to hold here.

1. We can and should decompose functions down into well named, single responsibility private functions to increase the readability.
2. Casting any into a type gives us no protection and this code is touching the piece that is making promises it isn't guaranteeing.

I'm inclined to hold the line on the 1 and accept that 2 is a bigger issue that deserves individual treatment, but both seem worth discussion / alignment.

[stocaaro]

I was surprised TS is cool with this, but it looks like models is any above and specified by the return type here, which doesn't give our customers any type interface protection, does it? How do we get guarantees that all of the typed index methods from the type will be populated at runtime?

[stocaaro]

Appreciate the updates. One nit suggestion, but otherwise happy to carry on.

[jimblanc]

Approving changes to aws-amplify and core.