AWS::Amplify::App with backend environments support

## 2. Scope of request
Currently, CloudFormation support for Amplify apps revolves around "frontend" resources (`AutoBranchCreationConfig`,`BuildSpec`,`Repository`...).

This issue requests support for:
* adding and configuring "backend" resources like backend environments
(currently they can be added [via CLI](https://awscli.amazonaws.com/v2/documentation/api/latest/reference/amplify/create-backend-environment.html))
* associating frontend branches to backend environments
(similar to what is already possible with `AWS::Amplify::Branch-PullRequestEnvironmentName` [docs](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-amplify-branch.html#cfn-amplify-branch-pullrequestenvironmentname)).

## 3. Expected behavior

It should be possible to define frontend branches (already supported), backend environments, and automatically associate the two.

## 5. Helpful Links to speed up research and evaluation

Current CloudFormation support: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/AWS_Amplify.html
AWS CLI commands for amplify: https://docs.aws.amazon.com/cli/latest/reference/amplify/index.html#cli-aws-amplify

## 6. Category (required) - Will help with tagging and be easier to find by other users to +1

Front-end Web & Mobile

## 7. Any additional context (optional)

This would ideally be used to expand AWS CDK's L1 and L2 constructs for AWS Amplify.

Read the current AWS Amplify CloudFormation resource coverage and the AWS CLI create-backend-environment and branch documentation first. Trace the existing AWS::Amplify::App and AWS::Amplify::Branch coverage, then identify the AWS CDK L1 and L2 construct entry points implicated by the request. Done means frontend branches, backend environments, and their association can be defined and supported.

Written by the indexing model from the issue text.