[POC] Introduce and enforce new stack structure

Problem

Issue number, if available:

Changes

Introduces defineStack for passing backend resources into (auth, data, etc.) and update DefineBackendProps to only take in backend stacks from defineStack. These changes would be part of the next major version, intermediate changes are in https://github.com/aws-amplify/amplify-backend/pull/2157.

The code using this solution looks like this:

import { defineAuth, defineData, defineFunction, defineStack } from '@aws-amplify/backend';

// define resources
const auth = defineAuth({ ... });
const data = defineData({ ... });
const function = defineFunction({ ... });

export const stack1 = defineStack('stack1', { auth, data, function });

// amplify/backend.ts
import { defineBackend } from '@aws-amplify/backend';

const backend = defineBackend({ stack1 });

// examples of accessing resources
backend.stack1.resources.stack
backend.stack1.resources.auth.resources.userPool
backend.stack1.resources.function.resources.cfnResources.cfnFunction
backend.stack1.resources.function.addEnvironment(...)

Corresponding docs PR, if applicable:

Validation

Checklist

[ ] If this PR includes a functional change to the runtime behavior of the code, I have added or updated automated test coverage for this change.
[ ] If this PR requires a change to the Project Architecture README, I have included that update in this PR.
[ ] If this PR requires a docs update, I have linked to that docs PR above.
[ ] If this PR modifies E2E tests, makes changes to resource provisioning, or makes SDK calls, I have run the PR checks with the run-e2e label set.

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

aws-amplify / amplify-backend