feat: initial implementation of a StackSet L2 construct

[corymhall]

See README.md

We don't have to publish this right away if we don't want to, but I have most of an L2 for StackSets that I want to get out there. Some things that we should do.

1. We now have a couple of places where we are creating a different type of "Stack" and Synthesizer. We should centralize this in Core.
2. This does not currently handle assets. I originally built this for cdk bootstrap which doesn't use assets. We should be able to add similar functionality that Service Catalog uses.

Fixes #

[otaviomacedo]

Some related questions:

1. Can multiple StackSetStacks be defined in the same scope?
2. Would it make sense to require that a StackSet be defined in the scope of a StackSetStack?
3. (if the answer to the above is "no") Is there a way to infer which StackSetStack is the correct one, instead of forcing the user to pass a template property?

[corymhall]

Can multiple StackSetStacks be defined in the same scope?
Would it make sense to require that a StackSet be defined in the scope of a StackSetStack?
(if the answer to the above is "no") Is there a way to infer which StackSetStack is the correct one, instead of forcing the user to pass a template property?

It's a little confusing, but I modeled this after the way Service Catalog works. The StackSet construct is the L2 for the CfnStackSet CloudFormation resource and it requires that you give it a CloudFormation Stack template. We can't just use a normal cdk.Stack because there are some limitations to StackSet stack templates that we need to account for. Long term I also want to have other ways of providing a template

template: StackSetTemplate.fromStackSetStack(stackSetStack);
template: StackSetTemplate.fromAsset(asset);
template: StackSetTemplate.fromInline('');
etc.