Fold the "assets" module, which includes the Staging construct that takes care of staging asset files into the cloud assembly during synthesis into "core". This is in order to allow implementing custom resources that leverage assets throughout the framework.

A subsequent commit will add a mini-framework for custom resources that leverages this capability.

…ssets

This commit folds the `CustomResource` and `NestedStack` types from `@aws-cdk/aws-cloudformation` into `@aws-cdk/core` in order to allow code in `core` and other lower layers to use capabilities such as nested stacks and custom resources.

This comes at a minor sacrifice to API fidelity: the provider's service token is for custom resources is now passed as a simple `string` instead of a strongly typed `ICustomResourceProvider`. But this is negligible for this type of resource given the high involvement users require to use it anyway. Additionally, the `NestedStack` class accepts a `notificationArns` as a `string[]` instead of an `sns.ITopic[]`. In both cases the API in `@aws-cdk/aws-cloudformation` (which is considered a stable module) remains unchanged with a compatibility layer added.

We took this opportunity to change the behavior of custom resources so that it won't pascal-case property names by default. This resolves #4896 and resolves #7035 and supersedes #7034.

The API in the aws-cloudformation module are still supported for backwards compatibility but marked as deprecated.

We have `true` in the compatibility layer

A helper for implementing simple node.js-based custom resource providers. This is a simpler framework from what is offered `@aws-cdk/custom-resources.Provider`, designed to enable implementing custom resources with minimal dependencies.

To that end, this helper uses `CfnResource` to define the AWS Lambda function and the low-level asset support that is now part of the core module.

It DOES NOT support:
- Arbitrary lambda function handlers, only node.js function.
- Asynchronous "isComplete" waiters (limited to 15min lambda timeout).

This is a precursor for implementing support for Open ID connect providers in the AWS IAM module, which is a very low-level module in our stack.

…ependency

cdk-integ depends on "cdk" which dev-depends on @aws-cdk/core

…ae/core-provider-2

Co-authored-by: Jonathan Goldwasser <jogold@users.noreply.github.com>

- Reorganize source directory structure
- Multiple updates to README and docs.

…der-2

Implements `iam.OpenIdConnectProvider` through a custom resource.

See README for details.

Related #5388
Related #3949
Related #6308

Co-authored-by: Niranjan Jayakar <nija@amazon.com>

Co-authored-by: Niranjan Jayakar <nija@amazon.com>

…der-2