@aws-cdk/aws-appsync-alpha: Generate TypeScript type definitions when using code-first schema approach

### Description

When using the code-first schema approach, it would be very helpful if CDK could generate TypeScript type definitions to match the generated GraphQL types, similar to how [GraphQL Nexus](https://nexusjs.org) does it.

### Use Case

This would be especially useful with Lambda data sources, since currently types need to be manually defined in order to use `AppSyncResolverHandler` and/or `AppSyncResolverEvent` typings from the `@types/aws-lambda` package.

### Proposed Solution

Similar to GraphQL Nexus, CDK could generate an e.g. `appsync.d.ts` file in the root of the project. And maybe the output path could be customized in `cdk.json` context.

### Other information

_No response_

### Acknowledge

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

Start by reviewing the @aws-cdk/aws-appsync-alpha code-first schema implementation and how cdk.json context is handled. Compare the desired generated declarations with GraphQL Nexus and define the output-path behavior and generated types that would count as done.

Written by the indexing model from the issue text.